TEXT-BOOK 

OF  THE 

MATERIALS  OF  ENGINEERING 


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TEXT-BOOK 

OF  THE 

MATERIALS  OF  ENGINEERING 


BY 
HERBERT  F.  IVfOORE 

RESEARCH   PROFESSOR   OF   ENGINEERING   MATERIALS,   ENGINEERING 

EXPERIMENT  STATION,   UNIVERSITY   OF  ILLINOIS,    MEMBER 

AMERICAN  SOCIETY   FOR  TESTING   MATERIALS, 

WITH  A  CHAPTER  ON  CONCRETE 

BY 
HARRISON  F.  GONNERMAN 

RESEARCH   ASSISTANT   PROFESSOR   IN   THEORETICAL  AND   APPLIED   MECHANICS, 

ENGINEERING   EXPERIMENT   STATION,   UNIVERSITY   OF  ILLINOIS, 

MEMBER  AMERICAN   SOCIETY   FOR  TESTING   MATERIALS, 

MEMBER   AMERICAN   CONCRETE   INSTITUTE. 


SECOND  EDITION 
FIRST  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW  YORK:    239   WEST  39TH  STREET 

LONDON:    6  &  8  BOUVERIE  ST.,  E.  C.  4 

1920 


.   COPYRIGHT,  1917,  1920,  BY  THE 
McGRA\v-HiLL  BOOK  COMPANY,  INC. 


THK     M  A  I*  1.  K     1'KKSS     YOKJC     1* 


PREFACE  TO  SECOND  EDITION 

The  principal  change  made  in  revising  this  text  for  the 
second  edition  is  in  the  chapter  on  concrete,  which  has 
been  entirely  rewritten  by  Prof.  Harrison  F.  Gonnerman. 
A  short  chapter  has  been  added  on  rubber,  leather,  and 
hemp  rope,  considerable  material  has  been  added  to  the 
chapter  on  strain  and  stress,  and  to  the  chapter  on  inspec- 
tion and  testing,  a  separate  chapter  has  been  given  to  the 
subject  of  specifications,  and  tables,  figures,  and  references 
have  been  thoroughly  revised. 

HERBERT  F.  MOORE. 

URBANA,  ILLINOIS, 
January,  20,   1920. 


419546 


PREFACE  TO  FIRST  EDITION 

The  object  of  this  text-book  is  to  furnish  a  concise  pres- 
entation of  the  physical  properties  of  the  common  mate- 
rials used  in  structures  and  machines,  together  with  brief 
descriptions  of  their  manufacture  and  fabrication.  The 
book  is  intended  primarily  for  use  in  technical  schools  in 
connection  with  courses  in  the  Mechanics  of  Materials, 
or  in  connection  with  courses  in  the  Materials  Testing 
Laboratory.  It  is  hoped,  however,  that  the  book  may 
prove  to  be  of  use  to  draftsmen,  inspectors,  machinists,  and 
others  who,  dealing  with  the  materials  of  engineering  in 
their  daily  work,  wish  to  become  familiar  in  an  elementary 
way.  with  the  properties  of  those  materials. 

The  text  is  distinctly  elementary  in  character,  and  for 
the  reader  who  may  wish  to  pursue  his  studies  further  there 
is  given  at  the  end  of  each  chapter  a  list  of  selected  refer- 
ences. The  books  and  periodicals  named  in  these  lists  will 
be  found  in  nearly  all  technical  school  libraries,  and  in  many 
city  libraries.  For  the  convenience  of  teachers  who  may 
use  this  book  as  a  text,  a  list  of  questions  on  the  various 
chapters  is  given  at  the  end  of  the  last  chapter. 

This  work  is,  of  necessity,  a  compilation  of  data  from 
various  sources,  and  the  author  has  endeavored  to  give 
credit  where  it  is  due.  He  acknowledges  his  indebtedness 
to  the  references  given  in  the  lists  and  to  the  various  indi- 
viduals who  have  assisted  him. 

HERBERT  F.  MOORE. 

URBANA,  ILLINOIS, 
August,  5,  1917. 


CONTENTS 

PAGE 
PREFACE ' v 


CHAPTER  I 

INTRODUCTORY 

Scope  of  Subject — General  Properties  of  Materials,  Strength — 
Stiffness — Elasticity  and  Plasticity— Toughness  and  Brittleness — 
Ductility  and  Malleability — Adaptability  to  Engineering  Con- 
struction and  Facility  in  Fabrication — Uniformity  and  Reliability- 
Hardness — Durability — Electric  and  Magnetic  Properties — Clas- 
sification of  Materials — Tests  of  Materials. 


CHAPTER  II 

STRAIN  AND  STRESS 10 

Strain,  Unit  Strain — Stress,  Unit  Stress — Hooke's  Law — Uni- 
formly Distributed  Stress,  Tension,  Compression,  Shear — Non- 
uniform  Stress  Distribution  —  Flexure  —  Torsion  —  Combined 
Stresses — Axial  Load  Combined  with  Shearing  Stress — Shear  in 
Beams — Long  Compression  Members,  Columns,  Pillars,  Struts — 
Lateral  Strain  under  Load,  Poisson's  Ratio — Effect  of  Lateral 
Strain  on  Strength,  Three  Theories  of  Failure  of  Materials — 
Elementary  Formulas  Involving  the  Consideration  of  Lateral 
Strain — Stress-Strain  Diagrams  for  Materials — Elastic  Limit,  Pro- 
portional Limit,  Yield  Point — Ultimate  Strength — Significance 
of  the  Elastic  Limit,  the  Proportional  Limit  and  the  Yield  Point — 
Behavior  of  Materials  in  a  Partially  Plastic  State — Effect  of  Stress 
Beyond  the  Yield  Point — Resistance  of  Materials  to  Impact — 
Stiffness,  Significance  of  the  Modulus  of  Elasticity — Coefficient 
of  Expansion,  Stresses  due  to  Temperature. 


CHAPTER  III 

THE  RESISTANCE  OF  MATERIALS  TO  REPEATED  STRESS 42 

Importance  of  Resistance  to  Repeated  Stress — Loss  of  Energy  Dur- 
ing Application  and  Release  of  Load — Mechanical  Hysteresis  at 
Low  Stresses — Localized  Stress  in  Structural  and  Machine  Mem- 
bers— Repeated  Stress  Tests — Effect  of  Range  of  Stress — Con- 
stants for  the  Exponential  Equations  for  Repeated  Stress — 
Diagrams  for  the  Exponential  Equations  for  Repeated  Stress — 
Wrought  Iron  versus  Steel — Effect  of  Rapidity  of  Repetition  of 

vii 


viii  CONTENTS 

PAGE 

Stress — Effect  of  Rest  on  Resistance  to  Repeated  Stress — Effect  of 
Sudden  Change  of  Outline  of  Member — Effect  of  Surface  Finish — 
Effect  of  Internal  Flaws  in  Structure — Service  Expected  from 
Various  Machine  and  Structural  Parts. 

CHAPTER  IV 

WORKING  STRESS;  FACTOR  OF  SAFETY;  SELECTION  OF  MATERIALS.    .    .     59 
Working  Stress — Consequence  of  Failure  of  Material — Factor  of 
Safety — Standard  Allowable  Working  Stresses — Working  Stresses 
for    Material    Subjected    to    Repeated    Loading — Materials    for 
Various  Classes  of  Machines  or  Structures. 

CHAPTER  V 

THE  MANUFACTURE  OF  PIG  IRON 70 

Occurrence  of  Iron  in  Nature — Ores  of  Iron — Mining  and  Prepara- 
tion of  Iron  Ore — Reduction  of  Ore  to  Pig  Iron — Fuel  for  the 
Reduction  of  Iron  Ore — Flux  Used  in  Reducing  Iron  Ore — The 
Blast  Furnace — Preheating  the  Blast,  Hot  Stoves — Production  of 
Pig  Iron — Utilization  of  Blast-furnace  Slag. 

CHAPTER  VI 

THE  MANUFACTURE  OF  WROUGHT  IRON 80 

Importance  of  Wrought  Iron — Definition  of  Wrought  Iron — The 
Puddling  Process — Characteristics  of  Wrought  Iron — Charcoal 
Iron. 

CHAPTER  VII 

THE  MANUFACTURE  OF  OPEN-HEARTH  STEEL 86 

General  Features — Basic  and  Acid-steel  Processes — The  Open- 
hearth  Furnace — Charging  the  Open-hearth  Furnace — The  Con- 
trol of  the  Open-hearth  Process — Recarburization  of  Steel — Other 
Types  of  the  Open-hearth  Furnace — Fuel  for  the  Open-hearth  Fur- 
nace— Arrangement  of  Open-hearth  Steel  Plants — Uses  and  Limi- 
tations of  Open-hearth  Steel. 

CHAPTER  VIII 

THE  MANUFACTURE  OF  STEEL  BY  THE  BESSEMER  PROCESS 94 

General  Features — The  Bessemer  Converter — Pig  Iron  for  the 
Bessemer  Process — The  Operation  of  the  Bessemer  Converter — 
Basic  Bessemer  Process — General  Quality  and  Use  of  Bessemer 
Steel — Duplex  Processes  of  Steel-making. 


CONTENTS  ix 

PAGE 
CHAPTER  IX 

CEMENTATION  STEEL,  CRUCIBLE  STEEL,  AND  ELECTRIC-FURNACE  STEEL.  101 
The  Cementation  Process — Cementation  Steel — Case-carbonized 
Steel — The  Crucible  Process — The  Electric  Furnace  for  Refining 
Steel — Duplex  and  Triplex  Processes  at  Steel  Making,  Using  the 
Electric  Furnace — Types  of  Electric  Steel  Furnaces — Electric 
Reducing  of  Iron  Ore. 

CHAPTER  X 

IRON  AND  STEEL  CASTINGS 109 

Cast  Iron;  the  Cupola — Air-furnace  Iron — Open-hearth  Furnaces 
for  Cast  Iron — Semi-steel — Gray  Cast  Iron,  White  Cast  Iron, 
Chilled  Cast  Iron — Malleable  Cast  Iron — Steel  Castings. 

CHAPTER  XI 

THE   MECHANICAL   TREATMENT  OF  STEEL;   ROLLING,   FORGING   AND 

PRESSING 115 

Uses  of  Rolled  Steel — Steel  Ingots — Defects  in  Steel  Ingots — 
Effects  of  "Pipes"  and  their  Prevention — Effects  of  Segregation 
and  its  Prevention — Effects  of  Honeycombing  and  its  Prevention — 
The  Rolling  Mill — Cold-rolled  and  Cold-drawn  Steel— Forging  and 
Pressing  Processes. 

CHAPTER  XII 

THE  CRYSTALLINE  STRUCTURE  OF  IRON  AND  STEEL  AND  ITS  SIGNIFI- 
CANCE; THE  HE  AT- TREATMENT  OF  STEEL;  WELDING 125 

The  Importance  of  the  Crystalline  Structure  of  Metals— Crystal- 
lization of  Pure  Iron— Solutions,  Solid  Solutions — Illustrations  of 
the  Action  of  Solutions,  Eutectics— The  Cooling  of  Iroji-carbon 
Alloys— The  Solidification  of  Cast  Iron— The  Cooling  of  Steel 
to  Solidification  and  after  Solidification — The  Critical  Tempera- 
ture of  Steel,  the  Recalescence  Point — Tempering  Steel — Grain 
Size  of  Iron  and  Steel — Annealing  Steel  to  Remove  the  Effects  of 
Overstress— The  Welding  of  Steel,  Types  of  Welds— Fusion 
Welding — Applications  of  Different  Types  of  Welds — Strength 
of  Steel  and  other  Metals  under  High  Temperatures. 


CHAPTER  XIII 

THE  EFFECT  OF  VARIOUS  INGREDIENTS  ON  THE  PROPERTIES  OF  IRON 

AND  STEEL;  CORROSION       143 

The  Importance  of  Chemical  Compositions  of  Iron  and  Steel — 
Commercial    Pure    Iron — Carbon — Silicon — Phosphorus — Sulphur 


x  CONTENTS 

PAGE 

— Manganese —  Nickel  —  Chromium  — Vanadium — Tungsten, 
Molybdenum  and  Cobalt — Copper — Titanium — The  Corrosion  of 
Iron  and  Steel — Strength  and  Ductility  of  Iron  and  Steel. 

CHAPTER  XIV 

THE  NON-FERROUS  METALS  AND  ALLOYS 152 

Importance  of  Non-ferrous  Metals — Copper — Uses  of  Copper — 
Physical  Properties  of  Copper — Aluminum — Uses  of  Alminum — 
Properties  of  Aluminum — Zinc — Properties  of  Zinc — Uses  of  Zinc ; 
Non-ferrous  Alloys — Copper-zinc  Alloys;  Brasses — Copper-tin 
Alloys;  Bronzes — "Season"  and  Corrosion  Cracking  of  Brass  and 
Bronze — Three-metal  Alloys — Alloys  of  Aluminum — Special 
Alloys — Bearing  Metals. 

CHAPTER  XV 

TIMBER 165 

Uses  in  Engineering  Construction — Principal  Varieties  of  Struc- 
tural Timber — Production  of  Timber  in  the  United  States — 
Seasoning  of  Timber — Shrinkage  of  Timber  During  Seasoning — 
Classification  of  Lumber — Uses  of  Timber — Structure  of  Wood — 
Strength  of  Timber — Elastic  Properties  of  Wood — Strength  of 
Large  Pieces  of  Timber — Effect  of  Moisture  on  the  Strength  of 
Timber — Time  Element  in  the  Strength  of  Timber — Relation  of 
Strength  and  Shrinkage  of  Timber  to  Density — Common  Defects 
in  Timber — The  Grading  of  Lumber — Veneer,  Plywood — Decay 
of  Wood — Preservatives  for  Timber — Preservative  Processes  for 
Timber— Uses  of  Treated  Timber— Strength  of  Treated  Timber. 

CHAPTER  XVI 

STONE,  BRICK  AND  TERRA-COTTA .187 

General  Uses  of  Building  Stone — Varieties  of  Building  Stone — 
Stone  Quarrying  and  Stone  Cutting — Masonry  Construction — 
Strength  of  Stone  and  of  Stone  Masonry — Burnt-clay  Products — 
Brick,  Terra-cotta  and  Tile — General  Process  of  Brick-making, 
Classification  of  Building  Brick — Paving  Brick  and  Firebrick — 
Terra-cotta — Drain  Tile  and  Sewer  Pipe — Strength  of  Porcelain 
and  Stoneware — Sand-lime  Brick — Strength  of  Brick  and  Terra- 
cotta and  of  Brick  Masonry  and  Terra-cotta  Masonry — Dur- 
ability of  Brick  and  of  Terra-cotta  Masonry. 

CHAPTER  XVII 

CEMENTING  MATERIALS:  GYPSUM,  LIME,  AND  NATURAL  CEMENT  AND 

PORTLAND  CEMENT  .  .    196 


CONTENTS  xi 

PAGE 

Cementing  Materials — Gypsum — Manufacture  of  Gypsum  Prod- 
ucts— Structural  Uses  of  Gypsum  Products — Gypsum  as  a  Fire- 
proofing  Material — Strength  of  Structural  Gypsum — Lime — 
Hydrated  Lime — Natural  Cement — Puzzolan  Cement — Portland 
Cement — Raw  Materials  for  Portland  Cement — Manufacture  of 
Portland  Cement. 

CHAPTER  XVIII 

CONCRETE 205 

Portland  Cement  Concrete — Plain  Concrete  and  Reinforced 
Concrete — Concrete  Aggregates — Undesirable  Ingredients  in 
Concrete  Aggregates — Proportioning  Aggregate  and  Cement  for 
Concrete — Proportioning  by  Arbitrary  Selection  of  Volumes — 
Proportioning  by  Trial  Mixtures — Proportioning  by  Voids  in 
Aggregate — Mechanical  Analysis  and  its  Application  to  the  Pro- 
portioning of  Concrete — Fuller  and  Thompson's  Method  of  Pro- 
portioning Concrete — Abrams'  Fineness  Modulus  Method  of 
Proportioning  Concrete — Design  of  Concrete  Mixtures  by  Abrams' 
Fineness  Modulus  Method — Edwards'  Surface  Area  Method  of  Pro- 
portioning Mortar  and  Concrete — Comparison  of  Methods  of 
Proportioning  Concrete — Mixing  Concrete — Handling  and  Placing 
Concrete — Curing  of  Concrete — Molds  and  Forms  for  Concrete — 
Strength  of  Concrete — Working'Stresses  in  Concrete — Effect  of  Low 
Temperature  on  Newly  Made  Concrete — Disintegration  of  Con- 
crete, Waterproofing — Use  of  Concrete  for  Fireproofing. 


CHAPTER  XIX 

RUBBER,  LEATHER  BELTING,  ROPE 254 

Rubber,  General  •  Characteristics — Production  of  Rubber — 
Physical  Properties  of  Rubber — Energy  Absorbed  by  Rubber  under 
Stress — Mechanical  "Hysteresis"  of  Rubber — Deterioration  of 
Rubber — Leather — Weight  and  Strength  of  Leather  Belting — 
Strength  of  Belt  Joints — Canvas  Belting — Rubber  Belting — Rope. 

CHAPTER  XX 

TESTING,  INSPECTION,  AND  TESTING  MACHINES 262 

Growing  Importance  of  Testing — The  Testing  Engineer — Defini- 
tion of  Terms — Commercial  Testing — Research  Testing — Testing 
Machines,  Tension — Compression — Flexure  Machines — Torsion 
Testing  Machines — Measurement  of  Strain,  Extensometers — 
Determination  of  the  "Elastic  Limit" — Impact  Tests  and  Impact 
Repeated  Stress  Tests  and  Testing  Machines — Hardness  Testing 
Apparatus — Cold-bend  Tests — Magnetic  Tests  of  Steel  as  an  Index 
of  Mechanical  Properties. 


xii  CONTENTS 

PAGE 

CHAPTER  XXI 

SPECIFICATIONS  FOR  MATERIALS 289 

General  Characteristics  of  Specifications  for  Materials — Summary 
of  Tests  Required  for  Materials. 

QUESTIONS 295 

INDEX  .   307 


TEXT-BOOK  OF  THE 
MATERIALS  OF  ENGINEERING 

CHAPTER  I 
INTRODUCTORY 

Scope  of  Subject. — The  study  of  the  materials  used  by 
engineers  in  the  construction  of  buildings,  bridges,  motors, 
machine  tools,  and  other  structures  and  machines 
includes  a  consideration  of:  (a)  the  methods  of  manufac- 
ture and  of  fabrication  into  structures  or  machines  of  the 
material  in  common  use;  (b)  the  properties  of  these 
materials;  (c)  the  requirements  of  service  which  these 
materials  must  meet ;,  and  (d)  methods  of  testing  and  in- 
spection which  are  used  to  insure  that  these  service  re- 
quirements shall  be  met.  Before  taking  up  a  detailed 
study  of  different  materials,  the  effects  on  materials  of 
stress  and  strain  will  be  discussed  in  a  general  way. 

General  Properties  of  Materials,  Strength. — For  all 
structures  and  machines  the  materials  used  must  have 
sufficient  strength  to  prevent  the  actual  breaking  of  parts 
and  the  consequent  failure  of  the  structure  or  machine. 
In  some  cases  strength  is  the  prime  requisite  of  the  mate- 
rials used  (e.g.,  bridges,  cranes,  punch  and  shear  frames) ; 
in  other  cases  strength  of  the  materials  is  a  secondary 
consideration.  The  strength  of  a  material  is  a  measure  of 
its  ability  to  resist  the  application  of  load  without  rupture, 
collapse,  or  undue  distortion.  The  application  of  load 
to  a  part  of  a  machine  or  structure  causes  internal  resisting 
forces  or  stress  to  be  set  up  in  that  part.  There  are  three 
kinds  of  elementary  stress:  tension  (e.g.,  the  stress  in  a  rope 

l 


-••.-         --  . 

:--  •          '''MATERIALS  OF  ENGINEERING 


holding  up  a  weight);  compression  (e.g.,  the  stress  in  the 
pillars  holding  up  a  floor);  and  shear  (e.g.,  the  stress  in  the 
rivets  splicing  together  two  plates  under  tension).  Fig.  1 
illustrates  these  elementary  stresses.  Flexure  or  bending 
stress  (e.g.,  the  stress  set  up  in  a  loaded  beam)  is  a  combi- 
nation of  tension  and  compression;  bearing  stress  (e.g., 
the  stress  on  the  floor  directly  under  the  legs  of  a  table)  is 
largely  compression;  and  torsion  (e.g.,  the  stress  in  a  shaft 
transmitting  power)  is  a  special  case  of  shear. 


.-Floor 


I 


^Compression 


.Shear 


FIG.  1.— Illustrations  of  elementary  stress. 

Stiffness. — Stiffness  and  strength  of  materials  are  some- 
times confused,  they  are,  however,  two  distinct  properties. 
If  any  stress  is  set  up  in  a  part  of  a  machine  or  structure 
the  form  of  that  part  is  slightly  changed.  This  change  of 
form  is  called  strain  or  deformation.  The  stiffness  of  a 
material  is  measured  by  the  magnitude  of  the  change  of 
form  under  stress.  Stiffness  is  frequently  a  very  important 
property  of  a  material.  For  example,  in  a  machine  tool 
there  should  be  very  slight  deflection  of  parts  under  normal 
conditions  of  working,  else  the  machine  tool  will  fail  to 
produce  work  of  a  sufficient  degree  of  precision.  In  this 
case  material  of  high  stiffness  is  desirable.  On  the  other 
hand,  it  is  desirable  that  railway  ties  should  yield  under 


INTRODUCTORY  3 

load  so  as  to  minimize  shock,  and  for  this  service  a  material 
with  a  low  degree  of  stiffness,  such  as  timber,  is  preferable 
to  one  with  a  high  degree  of  stiffness. 

Elasticity  and  Plasticity. — Materials  under  low  stresses 
do  not  suffer  any  appreciable  permanent  change  of  form, 
assuming  their  original  form  after  the  removal  of  load. 
Under  such  conditions  they  are  said  to  be  elastic.  Under 
high  stresses  materials  do  suffer  a  permanent  change  of 
form,  and  become  somewhat  plastic.  A  perfectly  plastic 
body  after  the  removal  of  load  would  retain  the  form  as- 
sumed under  load.  Under  increasing  load  the  change  from 
a  condition  of  nearly  perfect  elasticity  to  one  of  a  con- 
siderable degree  of  plasticity  ocurs  suddenly  for  some  ma- 
terials, especially  for  steel,  wrought  iron,  and  most  rolled 
.or  hammered  metals;  such  materials  are  said  to  have  a 
well-defined  yield  point.  Other  materials,  such  as  timber, 
concrete,  cast  iron,  do  not  become  plastic  to  any  noticeable 
extent.  For  nearly  all  engineering  purposes  it  is  desirable 
that  the  materials  used  should  remain  elastic  under  work- 
ing conditions. 

Toughness  and  Brittleness. — Some  materials  will  with- 
stand great  deformation  together  with  high  stress  without 
actual  rupture;  such  materials  possess  great  toughness. 
Other  materials  shatter  before  much  deformation  is  notice- 
able; such  materials  are  brittle.  Rolled  steel  and  rubber 
are  examples  of  tough  materials,  cast  iron  and  glass  of 
brittle  materials.  Toughness  is  a  highly  desirable  quality 
in  materials  for  structures  and  machines  in  which  sudden 
shattering  rupture  is  especially  disastrous;  e.g.,  it  is  quite 
necessary  that  car  couplers  and  car  frames  should  be  able 
to  resist  the  severe  shocks  of  service,  and  possibly  of  minor 
accidents,  without  actual  rupture,  even  if  considerable 
permanent  distortion  takes  place. 

Ductility  and  Malleability. — Some  materials  under  ten- 
sion will  suffer  a  considerable  elongation  before  actual  rup- 
ture takes  place,  such  materials  are  ductile.  A  ductile 
material  which  can  be  stretched  out  only  under  high  stress 
is  tough.  A  material  which  can  be  hammered  out  into  thin 


4  MATERIALS  OF  ENGINEERING 

sheets  is  malleable.  Lead  is  malleable  and  ductile,  but 
not  tough. 

Adaptability  to  Engineering  Construction  and  Facility  in 
Fabrication. — It  is  evident  that  even  material  possessing 
desirable  qualities  could  not  be  used  in  construction  if  it 
was  extremely  costly,  if  it  could  not  be  worked  into  the 
desired  shapes,  or  if  it  could  not  be  transported  to  or 
handled  at  the  place  where  the  structure  or  machine  was 
to  be  built. 

Uniformity  and  Reliability. — Some  materials  can  be 
produced  with  a  high  degree  of  uniformity  of  properties, 
such  as  strength,  stiffness,  etc.,  while  the  properties  of  other 
materials  can  not  be  foretold  within  wide  limits.  For 
engineering  work  material  which  is  uniform  in  quality  is 
always  desirable.  Materials  whose  structure  is  uniform 
throughout  resist  repeated  stress  better  than  materials  with 
non-uniform  structure.  Under  the  action  of  repeated 
stress,  minute  cracks  are  liable  to  start  at  points  where 
the  structure  is  not  uniform,  and  the  material  is  sometimes 
fractured  by  the  spread  of  these  minute  cracks. 

Hardness. — In  common  language  hardness  means  re- 
sistance to  abrasion.  Technically  the  term  is  usually 
used  to  denote  resistance  to  plastic  deformation. 

Durability. — It  is  desirable  that  during  the  period  of  use 
of  any  structure  or  machine  the  material  in  it  should  not 
deteriorate  in  quality.  Destructive  agencies  not  infre- 
quently act  on  the  materials  of  construction;  e.g.,  corrosion 
attacks  steel  and  iron,  bacterial  growths  cause  wood  to  de- 
cay, electrolytic  action  sometimes  destroys  concrete.  Me- 
chanical wear  of  parts  may  destroy  the  usefulness  of  a 
machine.  For  any  given  construction  the  durability  of 
the  materials  to  be  used  must  be  considered. 

Electric  and  Magnetic  Properties. — For  the  materials 
used  in  machines  and  structures  for  generating,  transmit- 
ting, and  utilizing  electric  energy  or  for  the  material  in 
structures  located  near  high-power  electric  circuits  the 
electric  and  magnetic  properties  are  frequently  of  prime 
importance. 


INTRODUCTORY  5 

Classification  of  Materials. — The  materials  of  construc- 
tion used  in  engineering  work  may  be  divided  into  two 
general  classes:  (a)  metallic,  and  (b)  non-metallic. 

Under  metallic  materials  we  may  classify: 

I.     THE  FERROUS  METALS-IRON  AND  STEEL 

1.  Cast  iron,  including  pig  iron,  cast  iron  in  castings, 
and  " semi-steel."     These  are  all  fusible,  but  not  malleable, 
not  weldable  (except  by  actual  fusion)  and  not  temperable ; 
they  all  contain  a  large  percentage  of  carbon  (high  carbon 
content) ,  are  brittle  and  have  a  crystalline  structure.     They 
are  used  in  cases  where  the  metal  is  to  be  cast  directly  into 
shape,  and  where  ductility  is  not  a  requisite.     Malleable 
cast  iron  is  cast  iron  with  its  carbon  content  and  crystalline 
structure  transformed  by  heating  and  slow  cooling  (anneal- 
ing).    It  has  a  considerable  degree  of  ductility. 

2.  Wrought  iron  is  manufactured  from  pig  iron  by  the 
"puddling"  process  without  fusion  of  the  final  product. 
It  has  a  fibrous  structure,  fibers  of  -slag  extending  through 
crystals  of  iron,  it  has  a  very  low  carbon  content,  it  is 
ductile,  weldable,  fusible  only  with  great  difficulty,  and  is 
not  temperable.     It  is  used  where  ease  of  welding  or  hot 
working  is  desirable,  e.g.,  in  small  blacksmith  shops,  and  is 
claimed  by  some  metallurgists  to  resist  corrosion  better  than 
steel.     Wrought  iron  is  widely  used  for  water  pipes. 

3.  Steel  is  manufactured  from  pig  iron  by  the  open-hearth 
process,  the  Bessemer  process,  or  the  electric  furnace  pro- 
cess,   or   by   a   combination  of  these  processes.     Special 
high  grades  of  steel  are  manufactured  from  wrought  iron 
by  the  crucible  process.     All  these  methods  involve  fusion. 
They  are  described  in  succeeding  chapters.     All  grades  of 
steel  are  fusible  and  malleable.     The  carbon  content  of 
commercial  steel  varidfr-from  almost  zero  to  1.25  per  cent. 
Steel  with  not  moje  than  0.25  per  cent,  carbon  is  mild 
steel;  0.25  to  0.60  per  cent,  carbon,  medium-carbon  steel; 
more  than  0.60  per  cent,  carbon,  high-carbon  steel.     Mild 
steel  and,  to  a  less  degree,  medium  steel  are  weldable,  non- 
temperable,  tough,  and  ductile.     Mild  steel  and  medium 


6  MATERIALS  OF  ENGINEERING 

steel  are  probably  the  most  important  metallic  materials 
of  engineering  construction.  They  are  used  in  all  kinds  of 
structural  and  machine  work.  High-carbon  steel  is  welda- 
ble  with  difficulty,  temperable  to  a  high  degree  and  of 
low  ductility.  It  is  used  for  tools,  springs,  and  in  cases 
where  great  strength  is  necessary. 

As  used  in  engineering  work  steel  may  be  cast  in  molds 
into  shape  (steel  castings)  or  into  blocks  (ingots)  of  cast 
steel  which  are  rolled  or  hammered  into  shape.  Rolled 
or  hammered  steel  is,  in  general,  stronger,  more  ductile 
and  tougher  than  are  steel  castings  of  the  same  chemical 
composition. 

Carbon  and  iron  are  the  principal  ingredients  in  steel, 
but  special  steels  of  great  strength  and  toughness  are  made 
by  alloying  carbon  and  iron  with  other  elements.  Com- 
mon alloy  steels  are:  nickel  steel,  chrome-nickel  steel, 
tungsten  steel,  vanadium  steel,  and  manganese  steel. 

II.    THE  NON-FERROUS  METALS 

1.  Copper. — Copper  is  manufactured  from  copper  ores, 
and  rolled   or   drawn  into   sheets   and  wires.     Both  its 
strength  and  its  ductility  are  somewhat  lower  than  the 
strength  and  the  ductility  of  steel.     Its  conductivity  for 
electric  currents  is  very  high.     The  principal  use  of  copper 
is  for  electric  wires,  and  it  is  used  to  a  limited  extent  for 
roofing,  pipes,  etc.,  where  resistance  to  corrosion  is  of  prime 
importance. 

2.  Aluminum. — Aluminum  is  manufactured  from  ores  by 
removal  of  oxygen  from  the  oxides  of  the  metals  (reduction) . 
Aluminum  is  the  lightest  metal  in  commercial  use:  rolled 
into  sheets  or  drawn  into  wires  it  has  considerable  strength, 
and  is  a  good  conductor  of  electricity.     It  is  used  for  small 
machine  parts  in  which  lightness  is  of  great  importance, 
and,  sometimes  alloyed  with  copper  it  is  used  for  electric 
wires,  especially  for  long  spans  on  electric  transmission 
lines.     Aluminum  resists  corrosion  and  is  used  for  kitchen 
ware  and  for  tanks  for  certain  chemicals. 

3.  Lead  Tin  Zinc.— Lead,  tin,  and  zinc  are  manufactured 


INTRODUCTORY  7 

from  ores  by  reduction.  These  metals  are  used  in  special 
cases  in  which  strength  is  not  a  prime  requisite  and  in 
which  resistance  to  chemical  action  is  necessary.  They  are 
used  in  alloys  for  bearings  for  shafting. 

4.  Brass. — Brass  is  an  alloy  of  zinc  and  copper.     It  is 
used  for  small  machine  parts  in  which  resistance  to  cor- 
rosion is  of  importance,  it  is  also  used  as  a  bearing  metal. 

5.  Bronze. — Bronze  is  an  alloy  of  copper  and  tin.     It  is 
used  where  resistance  to  corrosion  is  necessary,  and  where 
strength  is  also  necessary.     Bronze  may  be  made  almost 
as  strong  as  steel.     Bronze  is  used  as  a  bearing  metal  in  the 
highest  grade  of  bearings.     It  is  very  expensive  as  com- 
pared with  steel  and  is  used  only  in  special  cases.     Various 
alloys  of  copper,  tin,  zinc,  aluminum,  and  other  metals 
are  sometimes  spoken  of  as  " bronze"  and  are  used  for 
special  constructions. 

NON- METALLIC  MATERIALS 

1.  Wood. — Wood  is  very  easily  shaped  and  fastened  to- 
gether   for    temporary    construction.     When    new   or   if 
carefully  preserved  from  decaying,  wood  is,  in  most  cases, 
the  material  which  furnishes  a  given  strength  with  the 
least  weight.     It  is  used  for  house  and  ship  construction 
and  finish,  for  railway  ties,  temporary  trestles,  bridges  and 
other  structures,  for  patterns  for  castings,  and  forms  for 
concrete  construction.     Wood  frequently  deteriorates  with 
age  on  account  of  decay. 

2.  Brick   and    Terra-cotta.- — Brick   and   terro-cotta   are 
made  by  burning  clay.     They  are  used  widely  for  walls  and 
piers   of  buildings,   paving   and   drain  tile.     Terra-cotta 
tiles  are  used  for  roofs.     Brick  and  terra-cotta  are  brittle, 
but  possess  considerable  compressive  strength. 

3.  Concrete. — Concrete  is  made  by  mixing  some  cement- 
ing material,  usually  Portland  cement,  with  water  and  sand 
and  stone  (or  gravel) .     Concrete  can  be  poured  into  molds, 
which  are  removed  after  the  concrete  has  hardened.     Con- 
crete is  used  for  all  kinds  of  massive  structures.     It  has 
considerable  compressive  strength,  but  very  low  tensile 


8  MATERIALS  OF  ENGINEERING 

strength.  Where  tensile  strength  is  necessary,  steel  rods 
are  imbedded  in  the  concrete  to  take  the  tensile  stress,  this 
combination  being  known  as  reinforced  concrete.  Con- 
crete is  used  for  paving,  for  sidewalks,  for  walls,  floors, 
and  columns,  and  for  foundations  for  houses  and  machinery. 

4.  Mortar. — Mortar  is  made  by  mixing  cement  or  lime 
with  fine  sand  and  with  water.     It  is  used  as  a  surface  for 
walls,  for  the  outside  finish  of  houses,  and  for  cementing 
together  the  individual  pieces  of  brick  and  stone  work. 

5.  Gypsum. — Gypsum  when  mixed  with  water  hardens 
into  a  solid  mass  which  is  lighter  than  concrete  and  which 
possesses  less  strength.     Gypsum  is  used  for  light  walls 
and  for  roofing. 

6.  Natural  Stone. — Natural  stone  is  quarried  and  cut  to 
shape.     It  is  used  for  walls,  buildings,  and  bridges.     It  is 
regarded  as  the  most  nearly  permanent  form  of  construc- 
tion, but  is  very  expensive. 

7.  Rubber. — Rubber  is  made  from  the  juice  of  a  tropical 
tree.     Its  salient  characteristic  as  a  material  of  construction 
is  its  enormous  capacity  for  stretch,  and,  to  a  less  extent 
for    compression,    without    permanent    change    of    form. 
It  is  used  for  tires,  hose,  belting, and  " buffers"  for  absorb- 
ing the  energy  of  shock  in  machines. 

8.  Leather  and  Rawhide. — Leather  and  rawhide  are  made 
from  the  hides  of  animals,  the  former  being  put  through 
a  process  known  as  " tanning."     Leather  is  used  for  belts 
and  for  hydraulic  packings.     Rawhide  is  used  for  gears 
where  silent  operation  is  especially  desirable. 

9.  Hemp  and  Cotton. — Ropes  made  of  the  fibers  of  hemp 
and  cotton  are  in  very  common  use.     They  cannot,  of 
course,  be  used  for  carrying  any  stress  but  tensile  stress. 

Tests  of  Materials. — Chemical  Tests. — Chemical  analyses 
of  materials  whose  quality  is  under  investigation  are  usu- 
ally made  on  small  representative  samples  of  the  material 
and  have  for  their  chief  object  the  determination  of  the 
quantity  of  various  ingredients  present  in  the  material. 
They  are  frequently  used  to  detect  the  presence  of  an 
injurious  amount  of  an  undesirable  ingredient,  or  to  detect 


INTRODUCTORY  9 

unevenness  of  distribution  of  ingredients  through  a  mass  of 
material  (segregation) . 

Physical  Tests. — The  physical  tests  commonly  performed 
to  determine  the  quality  of  materials  of  construction  in- 
clude: tests  of  strength  (tension,  compression,  shear, 
bending,  torsion),  of  ductility  (elongation,  bending),  of 
brittleness  (impact  tests),  of  resistance  to  repeated  stress 
(endurance  tests),  of  hardness,  of  resistance  to  abrasion, 
and  of  the  internal  structure  (microscopic  examination). 
Physical  tests  are  made,  sometimes  on  a  small  sample  of 
the  material  and  sometimes  on  the  material  in  its  finished 
form.  Physical  tests  may  be  tests  to  destruction  (e.g.,  a 
tension  test  to  rupture  of  a  small  test  piece  made  from  a 
sample  taken  from  a  shipment  of  steel)  or  tests  under 
working  load  (e.g.,  a  test  of  the  floor  of  a  building  under  the 
load  it  is  supposed  to  carry  in  service).  Tests  to  destruc- 
tion, in  general,  give  relative  results  rather  than  absolute 
results  and  are  useful  in  determining  the  properties  of  a 
material  in  comparison  with  the  properties  of  material  of 
proven  quality  in  service. 


CHAPTER  II 
STRAIN  AND  STRESS 

Preliminary. — In  discussing  the  properties  of  materials 
it  is  necessary  to  refer  to  their  ability  to  withstand  deforma- 
tion and  to  resist  forces.  It  is  necessary  to  define  and  dis- 
cuss various  terms  which  are  used  in  connection  with  the 
behavior  of  materials  when  forces  are  applied  to  them  in 
order  that  these  terms  may  be  intelligible  when  used  in 
the  succeeding  chapters. 

Strain,  Unit  Strain. — Whenever  a  force  is  applied  to  any 
member  of  a  machine  or  a  structure — to  a  beam  in  a  house, 
a  shaft  in  a  machine  shop,  a  rail  in  a  railroad  track — -the 
shape  of  the  member  is  altered.  If  the  member  has  been 
properly  designed  to  withstand  the  force  the  change  of 
shape  is  small,  usually  not  directly  visible  to  the  eye;  on 
the  removal  of  the  force  the  member  returns  almost  exactly 
to  its  original  shape.  If  the  force  is  too  large  for  the  mem- 
ber to  carry  without  structural  injury,  there  usually  occurs 
a  considerable  change  of  shape.  If  this  overload  is  re- 
moved, the  body  does  not  return  to  its  original  shape,  but 
remains  permanently  distorted.  The  change  in  any  linear 
dimension  is  called  the  strain,  or  deformation,  and  the 
change  per  unit  of  linear  dimension  is  called  the  unit  strain 
or  unit  deformation  (measuring  in  inches  per  inch,  using 
the  units  common  in  engineering  practice). 

Stress,  Unit  Stress. — If  a  machine  part  or  structural 
member,  Fig.  2a,  is  acted  on  by  forces,  P,  P',  there  must 
be  set  up  within  the  body  internal  forces  called  stresses 
or  fiber  stresses  which  resist  the  tendency  of  the  external 
forces  to  tear  apart  or  to  crush  the  body.  Imagine  the 
part  of  the  body  at  one  side  of  any  section  mn  to  be  cut 
away,  then  to  represent  the  fiber  stresses  at  the  section 
mn  there  must  be  forces  S  to  balance  the  force  P  (Fig.  26). 

10 


STRAIN  AND  STRESS  11 

The  summation  of  the  forces  S  make  up  the  total  stress 
at  the  section  mn.  If  the  stress  over  a  very  small  portion 
of  the  section  is  denoted  by  A$  and  the  area  of  the  very 

small  section  is  denoted  by  &A,  then  for  that  small  area 
\   S» 
'•—T  is  the  intensity  of  stress,  or  the  stress  per  unit  area.  or. 

A  A. 

more*  briefly,  the  unit  stress.     If  the  stress  is  uniformly 

distributed  over  the  whole  area  of  cross-section  mn,  then 

p 
the  unit  stress  is-j  in  which  P  is  the  resultant  force  acting 

on  one  side  of  that  section,  and  A  is  the  area  of  the  section. 
Note  that  this  is  true  only  if  the  stress  is  uniformly  distrib- 


FIG.  2. — Machine  part  under  stress. 

uted.  In  general,  the  stress  per  unit  area  will  be  different 
at  different  locations  on  the  cross-section  mn.  The  mathe- 
matical analysis  of  stress  distribution  forms  the  subject 
matter  of  mechaincs  of  materials.  While  no  extended 
discussion  of  the  mathematical  analysis  of  stress  distribu- 
tion will  be  attempted  in  this  book,  there  are  given  in  this 
chapter  some  of  the  common  formulas  for  stress  and  strain, 
together  with  comment  on  their  limitations  and  application. 
Hooke's  Law. — Under  working  conditions  for  the  ma- 
terials commonly  used  to  carry  load  in  structures  and 
machines  stress  is  proportional  to  strain.  This  statement 
is  Hooke's  law,  and  is  named  after  the  English  scientist 
who  first  stated  it.  Hooke's  law  does  not  hold  for  high 


12  MATERIALS  OF  ENGINEERING 

stresses,  and  for  any  material  the  lowest  stress  for  which 
deviations  from  this  law  can  be  detected  is  an  index  of  the 
elastic  strength  of  the  material  (see  paragraph  following 
on  elastic  limit,  proportional  limit,  and  yield  point). 
Under  working  conditions  Hooke's  law  agrees  very  closely 
with  the  observed  action  of  rolled  metals,  and  also  of  steel 
castings;  it  is  a  fairly  close  approximation  for  cast  metals 
in  general,  for  concrete,  for  brick,  and  for  wood;  it  is  a 
rough  approximation  for  such  materials  as  rubber,  leather, 
and  hemp  rope.  The  mathematical  analyses  of  stress- 
distribution  commonly  used  by  engineers  assume  that 
Hooke's  law  holds,  and  do  not  apply  for  stresses  above  the 
(<  proportional  limit"  discussed  in  a  later  paragraph. 

Uniformly-distributed  Stress',  Tension,  Compression,  Shear. 
Under  axial  loading  the  stress  on  a  cross-section  of  a 
machine  or  structural  part  is  uniformly  distributed  across 
the  cross-section,  and  the  magnitude  of  the  unit  stress  is 
given  by  the  formula : 

8  =PA  (1) 

in  which  S  is  the  unit  stress  in  Ib.  per  sq.  in.,  P  is  the  axial  load  in  pounds, 
and  A  is  the  area  of  the  cross-section  in  sq.  in. 

Such  a  uniform  stress-distribution  is  assumed  for  the  bodies 
of  eyebars,  for  tie  rods,  for  bolts  under  tension,  for  ropes, 
for  belts,  for  guy  wires,  for  short  posts,  for  bearing  blocks, 
etc.  The  stress-distribution  in  long  compression  members 
is  discussed  in  a  later  paragraph.  In  considering  any 
actual  structural  or  machine  part  the  use  of  the  above 
formula  neglects  many  localized  stresses,  especially  near 
the  points  of  attachment  to  other  parts, — for  example  the 
bearing  stresses  between  an  eyebar  and  the  pin  to  which 
it  is  attached.  Except  for  parts  subjected  to  oft-repeated 
loading  these  localized  stresses  are  usually  not  important. 
A  fuller  discussion  of  the  significance  of  such  localized 
stresses  is  given  in  Chapter  III. 

For  parts  subjected  to  transverse  forces  whose  lines  of 
action  are  parallel  and  so  close  together  that  there  is  no 


STRAIN  AND  STRESS 


13 


appreciable  bending  action  (see  Fig.  3),  such  as  plates 
riveted  or  bolted  together  the  magnitude  of  the  shearing 
unit  stress  is  frequently  assumed  as: 

of  —    *  ( 9^ 

*^a  ~A~  \     ) 

in  which  Ss  is  the  shearing  unit  stress  in  Ib.  per  sq.  in.,  assumed  to  be 
uniformly  distributed  over  the  surface  of  the  cross-section;  P,  is  the 
shearing  load  in  pounds;  and  A  is  the  area  of  the  cross-section  in  sq.  in. 


Tension 


'P 
Compression 


(tough  Approximation) 
Shear 
FIG.  3. — Uniformly  distributed  stress. 


This  formula  is  a  rough  approximation  rather  than  an 
exact  statement.  There  are  always  present  heavy  bearing 
stresses  at  points  of  contact  of  adjacent  parts,  with  a 
resulting  cutting  action  at  the  surfaces  of  the  bolts  or 


FIG.  4. — Stress  on  oblique  section. 


rivets  and  an  intensified  shearing  stress.  However,  the 
foregoing  formula  is  fairly  reliable  for  computing  the  load 
necessary  to  shear  off  a  bolt,  rivet,  or  pin. 

In  any  machine  or  structural  member  under  axial  load, 


14  MATERIALS  OF  ENGINEERING 

on  any  plane  oblique  to  the  action  line  of  the  load  there  are 
both  tensile  (or  compressive)  and  shearing  stresses  (see 

Fig.  4) .     For  any  oblique  plane  the  tensile  (or  compressive) 

p 
unit  stress  is  less  than  -r.     The  shearing  unit  stress  is 

maximum  for  a  plane  at  45  degrees  with  the  axis;  for  this 

p 
plane  the  shearing  unit  stress  is  0.5  -T-. 

Non -Uniform  Stress  Distribution. — Under  flexure,  under 
torsion,  or  under  combinations  of  flexure,  torsion,  and  axial 
loading  the  stress-distribution  in  a  structural  or  machine 
part  is  more  complex  than  under  axial  load  alone.  On  any 
cross-section  the  unit  stress  varies  from  surface  to  surface, 
or  from  axis  to  surface,  and  the  study  of  the  stress-distri- 
bution has  for  its  main  object  the  determination  of  the 
maximum  unit  stress  existing  in  the  part,  or,  as  it  is  some- 
times put,  the  location  of  the  most-stressed  fiber,  and  the 
determination  of  the  unit  stress  in  it. 

Flexure. — In  a  machine  or  structural  part  under  cross- 
bending  or  flexure  the  external  bending  moment  at  any 
cross-section,  caused  by  the  loads,  -reactions,  and  couples 
at  one  side  of  the  section  is  balanced  by  the  internal  mo- 
ment of  the  fiber  stresses.  The  stress  varies  from  tension 
at  one  edge  of  the  member  to  compression  at  the  opposite 
edge.  On  an  axis  passing  through  the  center  of  gravity 
(centroid)  of  the  cross-section  the  stress  is  zero,  and  within 
the  proportional  limit  the  unit  stress  in  any  fiber  is  pro- 
portional to  the  distance  of  the  fiber  from  this  neutral 
axis  as  it  is  called.  For  any  fiber  the  stress  is  given  by  the 
formula : 

,-[':,<•-•   s  =  f  :  (3) 

in  which  S  is  the  unit  stress  in  Ib.  per  sq.  in.  (tension  for  the  side  of  the 
member  made  convex  by  flexure,  compression  for  the  side  made  con- 
cave); M  is  the  bending  moment  in  inch-pounds  of  the  forces  and 
couples  on  one  side  of  the  section;  v  is  the  distance  in  inches  of  the  fiber 
from  the  neutral  axis;  and  7  is  the  so-called  "moment  of  inertia"  of 
the  cross-section  about  the  neutral  axis  in  (inches).4 


STRAIN  AND  STRESS 


15 


Type    of  Beam 


Magnitude 

of 

Maximum)  Maximum 
Shear      Shear 


Location 
of 


Shear 
Diagram 


.occrion 
of  Max. 

Jendi 
Home 


nfBend 


Magnitude 
of  Maximum 
inq  Moment 


Bending 
Moment 
Diagram 


I 


T 


A+oB 


PI 


_  ,  Total  Load-  W 

mwmjiiiiuiiiimiuiii 

™=? 


W 


Wl 

2 


...  t  .. 


AteC 
CbB 


" 


Ht-.~-  I  ----- 


AioC 
or 

CtoB 


-^-PfromAtoC 


ri 


mp.-' 


nmp 


AtoC 


CfcD 


Pa 


Total  Load-  W 


mniinn 


A&B 


Wl 

3 


or 

CteA 


from  BfoC 


fromC-hoA 


^ 


Total  Load-  W 


..f- 


(Shear  at 


(Moment  a+C 


AtoC 
and 

CtoB 


A,B, 

and 

C 


PL 


AtoC 
or 

CioB 


,  Total Load=  W 


from  A-teC 


•FromC-l-oB 


A,B 
or 
C 


A*g|j 

f 


A&B 


A  and 
5 


Moment  at  C 


FIG.  o. — Bending  moments  and  shears  for  beams. 


16 


MATERIALS  OF  ENGINEERING 


For  the  fiber  farthest  distant  from  the  neutral  axis  this 
formula  becomes: 

Me 


S 


(3a) 


in  which  c  is  the  distance  of  the  most  remote  fiber  from  the  neutral 
axis. 

In  using  formulas  (3)  and  (3a)  it  is  necessary  to  compute 
M  the  moment  at  the  cross-section  of  the  forces  and  couples 
on  one  side  of  the  cross-section  (either  side  may  be  taken) . 
If  the  forces  acting  on  the  member  can  all  be  determined 
by  the  common  equations  of  statics1  this  is  a  simple  mat- 


h 

x!- 


of  Grayitu, 


of  Gravity. 


Ce 


FIG.  6. — Moment  of  inertia  for  circle,  rectangle,  and  triangle. 

ter  and  the  member  is  said  to  be  " statically  determinate." 
If  the  forces  acting  cannot  be  so  readily  found,  as  for  exam- 
ple in  the  case  of  a  beam  with  more  than  two  supports, 
the  member  is  said  to  be  " statically  indeterminate," 
and  to  determine  the  moment  at  any  cross-section  the 
elastic  deformation  of  the  member  must  be  considered. 
Fig.  5  gives  the  magnitude  and  location  of  maximum 
bending  moments  for  a  number  of  common  cases  of  flexure. 
In  using  formula  (3a)  it  is  necessary  to  determine  the 
value  of  /  and  of  c  for  each  case  under  consideration. 
Fig.  6  gives  the  value  of  7,  of  c,  and  of  I/c  (called  the  sec- 
tion modulus)  for  the  circle,  the  rectangle,  and  the  triangle. 
The  value  of  I/c  for  more  complicated  shapes  can  be  com- 

1  Sum  of  vertical  forces  for  whole  body  =  0. 
Sum  of  horizontal  forces  for  whole  body  =  0. 
Sum  of  bending  moments  for  whole  body  =  0. 


STRAIN  AND  STRESS  17 

puted  by  dividing  the  shape  into  approximate  rectangles 
and  triangles.  First  the  center  of  gravity  of  the  composite 
figure  is  found,  then  the  fiber  most  remote  from  the  axis 
drawn  through  the  center  of  gravity  perpendicular  to  the 
plane  of  bending  can  be  determined  by  inspection,  and 
c  measured  directly.  The  I  of  the  composite  section  is  the 
sum  of  the  7's  of  the  component  rectangles  and  triangles 
about  the  neutral  axis  of  the  whole  section.  For  any  com- 
ponent rectangle  or  triangle  the  /  about  the  neutral  axis 
is  given  by  the  formula : 

I  =  I  +  Ad2  (4) 

in  which  /  is  the  moment  of  inertia  of  the  rectangle  or  triangle  about 
its  own  centroidal  axis  (see  Fig.  6) ;  A  is  the  area  of  the  rectangle  or 
triangle;  d  is  the  distance  from  the  center  of  gravity  of  the  rectangle  or 
triangle  to  the  neutral  axis  of  the  whole  figure;  and  I  is  the  moment  of 
inertia  of  the  rectangle  or  triangle  about  the  neutral  axis  of  the  com- 
posite figure. 

Fig.  7  shows  the  working  out  of  this  method  for  deter- 
mining I  and  I/c  of  a  cross-section  of  a  punch  frame. 

Special  graphical  methods  for  determining  the  value  of 
7  for  irregular  sections  are  given  in  many  texts  on  mechan- 
ics of  materials. 

Formulas  (3)  and  (3a)  are  true  only  when  a  "  principal 
axis"  (axis  for  which  the  I  of  a  cross-section  is  a  maximum 
or  a  minimum)  lies  in  the  plane  of  the  loads  causing  the 
bending.  This  is  the  case  for  members  whose  cross-sec- 
tions have  an  axis  of  symmetry  either  in  the  plane  of  bend- 
ing or  at  right  angles  to  that  plane.  The  large  majority 
of  flexure-resisting  members  are  loaded  so  as  to  fulfil  this 
condition.  Fig.  8  shows  several  such  sections  together 
with  some  which  do  not  fulfill  the  above  condition,  and  for 
which  formulas  (3)  and  (3o)  cannot  be  used.  The  stresses 
for  these  unusual  cases  may  be  widely  different  from  the 
stresses  given  by  the  use  of  formulas  (3)  and  (3o) .  Exam- 
ples of  such  obliquely  loaded  flexure  members  are:  roof 
purlins,  and  railroad  rails.  In  this  connection  attention 

is  called  to  the  references  given  at  the  end  of  this  chapter. 
2 


18 


MATERIALS  OF  ENGINEERING 


For  beams  whose  axes  are  curved  lines,  such  as  hooks 
elliptical  springs,  chain  links,  etc.,  formulas  (3)  and  (3a) 
are  inexact.  The  neutral  axis  is  between  the  center  of 
gravity  of  the  cross-section  and  the  concave  side  of  the 
curved  beam,  the  unit  stress  on  the  concave  side  is  increased 

PROBLEM. — Find  the  distance  y  of  the  gravity 
axis  XX  from  the  base  PN  of  the  cross-section 
shown,  and  find  I  and  I/c  for  the  cross-section. 

The  composite  figure  is  divided  into  elementary 
rectangles  and  triangles,  A,  B,  B',  C,  C',  and  D. 
The  area  of  each  elementary  part  and  moment  of 
that  area  about  PN  are  determined.  Then  y  is 
equal  to  the  sum  of  the  elementary  moments 
divided  by  the  sum  of  the  elementary  areas. 
The  sum  of  the  moments  of  inertia  about  the  axis 
XX  of  the  elementary  parts  gives  the  moment  of 
inertia  of  the  whole  cross-section. 

For  each  elementary  part,  I  =  I  +  Ad2 


FIG.  7. — Moment  of 
inertia  for  composite 
figure. 


Distance 

Distance 

/of 

of  center 

Moment 

of  center 

part 

Part 

Width 
(in.) 
b 

Height 
(in.) 
h 

Area 

(sq. 

5 

of  grav- 
ity of 
part 
from  PN 

of  part 
about 
PN 
(in.)  s 

of  grav- 
ity of 
part 
from  XX 

d2 
(sq. 
in.) 

A3* 

(in.)  « 

about 
its  own 
gravity 
axis 

(in.) 

M  -  Aq 

(in.) 

(in.)4 

« 

4 

I 

A 

8.0 

2.0 

16.0 

1.00 

16.00 

-3.19 

10.18 

162.9 

5.3 

B 

1.0 

7.0 

7.0 

5.50 

38.50 

+  1.31 

1.72 

12.0 

28.6 

B' 

1.0 

7.0 

7.0 

5.50 

38.50 

+  1.31 

1.72 

12.0 

28.6 

Cf 

1.0 

7.0 

3.5 

4.33 

15.17 

+0.14 

0.02 

0.1 

9.5 

C' 

1.0 

7.0 

3.5 

4.33 

15.17 

+0.14 

0.02 

0.1 

9.5 

D 

6.0 

1.0 

6.0 

9.50 

57.00 

+  5.31 

28.20 

169.2 

0.5 

Sum  of  elementary  areas  (SA)  =  43.0 

Sum  of  elementary  moments  (SM)  =  180.34 

Distance  of  center  of  gravity  of  whole  cross-section  from  PN  — 


Sum  of  Ad2  (2  Ad2)  =  356.3 

Sum  of  7's  (SI)  =  82.0 

I  for  whole  cross-section  =  356.3  +•  82.0  =  438.3  (in.)4 

c  =  10.00  -  4.19  =  5.81  in. 


4.19  in. 


I/c  = 


=  75.4  (in.)3 


and  the  unit  stress  on  the  convex  side  is  decreased.  The 
less  the  radius  of  curvature  of  the  axis  of  the  beam  the  greater 
is  the  variation  of  unit  stresses  from  the  values  given  by 


STRAIN  AND  STRESS 


19 


formula  (3a) .  In  this  connection  attention  is  called  to  the 
special  references  given  at  the  end  of  the  chapter.  For- 
mula (3a)  is,  however,  frequently  used  to  give  a  rough 
approximation  for  stress  in  curved  beams. 

There  are  shearing  stresses  in  members  subjected  to 
flexure,  and  the  distribution  of  these  shearing  stresses  is 
discussed  briefly  in  a  succeeding  paragraph. 

Formula  (3a)  is  sometimes  used  to  give  comparative 
values  for  tests  of  materials  to  fracture.  The  value  of 
S  determined  from  the  load  at  fracture  for  a  flexure  test 
is  called  the  Modulus  of  Rupture. 


Load       Load 


Load 


Load 


Ax  is  of 
Symmetry 


Load 


.* :.  >:i 


Symmetrical  Loading-  Formula  (3a) Holds 


-.>.  Unsymmetrical  Loading 

For  the  above  Cases 
Formula  (3a)does  not  Hold 

FIG.  8.  —  Symmetrical  and  non-symmetrical  loading  for  beams. 

Torsion.  —  The  formula  commonly  used  for  computing 
shearing  unit  stresses  in  structural  and  machine  parts  under 
torsion  applies  only  within  the  proportional  limit  and  only 
to  members  whose  cross-section  is  either  a  circle  or  a  hol- 
low circle.  For  such  members  the  fiber  stress  is  a  shearing 
stress  varying  from  zero  at  the  axis  of  the  member  to  a 
maximum  at  the  surface.  At  any  point  in  the  member 
there  is  a  shearing  unit  stress  parallel  to  the  axis  and  a  shear- 
ing unit  stress  of  equal  magnitude  perpendicular  to  the  axis. 
These  stresses  are  called  the  longitudinal  shearing  stress 
and  the  transverse  shearing  stress  respectively.  The 
relation  between  twisting  moment  and  maximum  shearing 
unit  stress  (either  longitudinal  or  transverse)  is  given  by 
the  formula  : 


S. 


(5) 


20  MATERIALS  OF  ENGINEERING 

in  which  T  is  the  twisting  moment  in  inch-pounds  on  one  side  of  the 
cross-section;  S,  is  the  maximum  shearing  unit  stress  in  Ib.  per  sq.  in. 
J  is  the  polar  moment  of  inertia  of  the  cross-section  in  (inches)4;  and 
c  is  the  radius  of  the  cross-section. 

Values  of  J  and      for  a  circle  and  for  a  hollow  circle  are 
c 

given  in  Fig.  9.  The  twisting  moment  at  any  cross-sec- 
tion of  a  member  under  torsion  is  given  by  the  algebraic 
sum  of  the  twisting  moments  on  one  side  of  that  section 
(either  side  may  be  considered).  For  a  shaft  transmitting 
hp  horse  power  at  N  revolutions  per  minute  the  twisting 
moment  in  inch-pounds  is  63000  hp/N. 

The  shearing  stresses  in  a  shaft  of  non-circular  cross- 
section  are  very  complex.     Approximate  values  for  maxi- 


J/c-%d3 
FIG.  9. — Polar  moment  of  inertia  for  circle  and  hollow  circle. 

mum  shearing  unit  stress  for  various  cross-sections  are 
given  in  Fig.  10  these  values  are  based  on  the  experimental 
work  of  Bach  and  of  Kommers. 

In  using  formula  (5)  or  the  formulas  given  in  Fig.  10  it 
must  be  borne  in  mind  that  the  unit  stress  determined  is  a 
shearing  stress  and  that  for  any  material  the  safe  shearing 
unit  stress  is  different  from  the  safe  tensile  or  compressive 
stress.  This  consideration  is  further  discussed  in  Chapter 
IV.  On  any  oblique  cross-section  of  a  member  under 
torsion  there  are,  in  general,  both  shearing  stresses  and 
tensile  (or  compressive)  stresses  (see  Fig.  11).  On  a 
cross-section  perpendicular  to  the  axis  or  in  a  direction 
parallel  to  the  axis  the  shearing  stress  reaches  its  maximum 
value,  and  the  tensile  (or  compressive)  stress  is  zero.  On 
a  45-degree  section  the  shearing  stress  is  zero,  and  the 


STRAIN  AND  STRESS 


21 


tensile  (or  compress! ve)  stress  reaches  a  maximum  value. 
On  a  45-degree  section  the  value  of  the  unit  tensile  (or 
compressive)  stress  is  equal  to  the  value  of  the  unit  shear- 

S$ *=   T/Z  in  which  SQ  =  max.  unitshearing  stress  (Ib.per  sq.  in.); 
T=  twisting  moment-  (inch-pounds)  ,•  Z"  forsionat 
section  modulus.. 

The  ya  lues  of  Z  given  in  this  figure  are  based  on  the  experimental  work  of 
Bach  and  of  Kommers. 


Cross  -section 


z(in  inches J 


Cross -sect  ion 


z(in  inches) 


Cross-section 


Z(in  inches) 


T~ 
h 

1_ 


t><h 


20 


1.09 


it2** 


*  Kommers' Tests  indicate  that  the  flanges  of  I  and  channel  sections  add  very 
littletothevalueofZ. 

FIG.   10. — Torsion  constants  for  non-circular  sections. 

ing  stress  on  a  cross-section  at  right  angles  to  the  axis  of 
the  member:  that  is  for  a  circular  shaft  on  a  45-degree 
section  the  maximum  tensile  (or  compressive)  unit  stress 

Tc 
is  equal  to  -j-  •     This  is  of  im- 

portance   in  computing  unit 
stresses   for  members  in  torsion 

Which  are  made  Of  materials  Which     FIG.  11  -Stresses  on  oblique  cross- 
section  of  a  torsion  member. 

are  weaker  in  tension  than  they 

are  in  shear,  such  as  cast  iron  and  most  brittle  materials. 

For  such  materials  under  torsion  no  computed  unit  shearing 


22  MATERIALS  OF  ENGINEERING 

stress  should  be  allowed  greater  than  the  safe  unit  stress  in 
tension. 

Combined  Stresses.  —  The  formulas  given  in  the  following 
paragraphs  on  combined  stress  are  based,  as  are  the  pre- 
ceding paragraphs,  on  the  common  theory  of  elastic  action. 
These  formulas  serve  satisfactorily  for  general  practice, 
although  somewhat  more  exact  formulas  have  been  de- 
veloped that  taken  into  account  the  lateral  strain,  which 
always  accompanies  stress.  These  more  exact  formulas 
are  very  briefly  treated  in  succeeding  paragraphs  on 
lateral  strain  under  load  and  on  Poisson's  ratio. 

Axial  Load  Combined  with  Flexure.  —  Axial  load  and 
flexure  in  a  member  set  up  stresses  which  act  in  the  same 
direction  or  in  opposite  directions,  hence  the  resulting 
combined  unit  stress  may  be  determined  by  adding  alge- 
braically the  stresses  caused  by  each  action.  The  unit 
stress  at  the  extreme  fibers  due  to  the  combination  of 
axial  load  and  flexure  is  : 

a      P    .Me 
8  '-  A  ±~T 
in  which  the  nomenclature  is  the  same  as  for  formulas  (1)  and  (3a). 

For  the  fibers  along  which  the  axial  stress  and  the  stress 
due  to  flexure  are  in  the  same  direction  the  plus  sign  is 
used;  for  the  fibers  along  which  the  axial  stress  and  the 
stress  due  to  flexura  are  in  opposite  directions  the  minus 
sign  is  used.  A  common  case  of  combined  axial  stress 
and  flexural  stress  occurs  in  a  member  which  is  subjected 
to  a  load  parallel  to  the  axis  at  a  distance  v  from  that  axis. 
For  such  a  member  M  is  equal  to  Pe,  and  formula  (6) 
becomes: 


If  r  denotes  the  radius  of  gyration  of  the  cross-section, 
measured  in  inches,  /  =  Ar*  and  the  above  equation  may 
be  written  : 


For  long  bars  at  the  center  of  their  length  M  differs  from 


STRAIN  AND  STRESS  23 

Pe  on  account  of  the  deflection  of  the  bar  under  eccentric 
load,  and  formula  (7)  becomes  inexact.  For  long  bars 
under  tension  the  deflection  diminishes  the  value  of  e  at 
the  middle  of  the  length,  and  consequently,  Pe  and  S 
are  diminished,  and  formula  (7)  errs  on  the  side  of  safety. 
For  compression  members  the  bending  increases  c  and 
consequently  Pe  and  S,  and  formula  (7)  errs  on  the  side 
of  danger.  For  bars  in  compression  whose  length  is  more 
than  four  or  five  times  the  smallest  dimension  of  the  cross- 
section  formula  (7)  should  not  be  used  (see  succeeding 
paragraph  on  long  columns) . 

Tensile  (or  Compressive)  Stress  combined  with  Shearing 
Stress.1 — When  there  is  present  in  a  machine  or  struc- 
tural member  tensile  stress,  caused  either  by  axial  load 
or  by  flexure,  and  at  the  same  time  there  is  present  shear- 
ing stress,  caused  either  by  direct  shear  or  by  torsion  there 
are  set  up  both  tensile  stresses  and  shearing  stresses  in 
the  member.  The  maximum  tensile  unit  stress  occurs,  in 
general,  on  some  oblique  plane,  and  on  that  plane  the 
unit  shearing  stress  is  zero:  the  mimimum  unit  tensile 
stress  occurs  on  a  plane  at  right  angles  to  that  for  the 
maximum  unit  tensile  stress,  and  on  the  plane  of  minimum 
unit  tensile  stress  the  unit  shearing  stress  is  also  zero  (it 
must  be  borne  in  mind  that  compressive  stresses  are  re- 
garded as  negative  tensile  stresses  in  this  section  and: 
minimum  tensile  stress  may  have  a  negative  value,  that 
is  may  be  a  compressive  stress.)  The  maximum  shearing 
unit  stress  occurs  on  some  oblique  plane  between  the  plane 
for  maximum  unit  tensile  stress  and  the  plane  for  minimum 
unit  tensile  stress.  The  maximum  unit  tensile  stress  is 

given  by  the  formula: 

St   , 


(8) 

The  maximum  unit  shearing  stress  is  given  by  the  formula: 

(9) 


1  In  this  paragraph  compressive  stress  is  regarded  as  negative  tensile 
stress,  and  the  terms  maximum  and  minimum  are  used  algebraically. 


24  MATERIALS  OF  ENGINEERING 

In  formulas  (8)  and  (9)  Sl  is  the  maximum  tensile  (or  compressive)  unit 
stress  resulting  from  the  combined  stress,  measured  in  Ib.  per  sq.  in.; 
St  is  the  tensile  (or  compressive)  unit  stress  caused  by  flexure  or  axial 
stress  on  the  fiber  under  consideration,  measured  in  Ib.  per  sq.  in.; 
Ss  is  the  shearing  unit  stress  caused  by  direct  shear  or  by  torsion  on  the 
fiber  under  consideration,  measured  in  Ib.  per  sq.  in.;  and  -S's  is  the 
maximum  shearing  unit  stress  resulting  from  the  combined  stress, 
measured  in  Ib.  per  sq.  in. 

In  using  formulas  (8)  and  (9)  it  must  be  borne  in  mind 
that  the  allowable  unit  shearing  stress  for  any  material 
is  different  from  the  allowable  unit  stress  in  tension  or 
compression.  Computation  may  show  S1,  to  be  less  than 
S1,  yet  if  the  material  is  weaker  in  shear  than  in  tension, 
as  is  the  case  with  most  ductile  metals,  the  danger  of  failure 
by  shear  may  be  greater  than  the  danger  of  failure  by 
tension.  In  general,  both  Sl  and  S1,  should  be  computed, 
and  the  safety  of  the  member  determined  both  against 
shearing  failure,  and  against  failure  by  tension  or  com- 
pression. 

Shear  in  Beams. — Beams  are  subjected  to  shearing  action 
as  well  as  to  flexure.  The  shearing  stress  for  any  cross- 
section  of  a  beam  is  computed  by  equating  the  summation 
of  the  internal  shearing  stresses  on  that  cross-section  to  the 
external  shear  set  up  by  the  loads  acting  on  one  side  of  that 
section  (either  side  may  be  considered).  At  any  point 
on  the  cross-section  of  the  beam  there  are  shearing  unit 
stress  in  the  plane  of  the  loads  and  perpendicular  to  the 
axis  of  the  beam  is  equal  in  magnitude  to  the  shearing  unit 
stress  in  the  plane  of  the  loads  and  parallel  to  the  axis  of  the 
beam,  or,  as  it  is  frequently  expressed,  the  vertical  shearing 
unit  stress  is  equal  in  magnitude  to  the  horizontal  shear- 
ing unit  stress.  The  magnitude  of  the  vertical  or  the 
horizontal  shearing  unit  stress  at  any  point  of  a  cross-section 
of  a  beam  is  given  by  the  formula : 

*S.*'-j£vA  (10) 

in  which  S8  is  the  shearing  unit  stress  at  the  given  point,  measured  in 
Ib.  per  sq.  in.;  V  is  the  shear  at  the  section,  measured  in  pounds,  A  is 


STRAIN  AND  STRESS 


25 


the  area  of  that  part  of  the  cross-section  between  a  line  passing  through 
the  given  point  parallel  to  the  neutral  axis  of  the  section  and  the  near- 
est extreme  fiber  of  the  cross-section  (see  Fig.  13) ;  v  is  the  distance  of 
the  center  of  gravity  of  the  above  part  of  the  cross-section  from  the 
neutral  axis  (vA  is  the  " static  moment"  of  the  above  part  of  the  cross- 
section  about  the  neutral  axis,  measured  in  (inches)3);  /  is  the  moment 
of  inertia  of  the  cross-section  in  (inches)4;  and  b  is  the  width  of  the 
cross-section  at  the  point  considered. 

Sa  is  evidently  zero  at  the  extreme  fiber  where  A  is  equal 
to  zero:  for  many  forms  of  beams,  including  rectangular 
beams,  circular  beams,  and  beams  of  I,  H,  T,  and  channel 
section,  Ss  is  a  maximum  at  the  neutral  axis,  but  this  is 
not  true  for  all  sections  of  beams.  Fig.  12  shows  the  varia- 


•ip^ljipj^^ 


!<-/,  ->l   kA^>l 

" S&J  x  X  is  the  gravitu  axis  Ratio  S5  at  mid-  height 

k  is  a  max.  at  mid- height      to  fV+  area  of  Web)  - 1. 045 

In  the  above  figures  K  is  the  ratioofSg  as  given  by  formula(io)  to  the  average  shearinq  unit 
stress  for  the  whole  cross-section  (V/A) 

FIG.  12. — Distribution  of  shearing  stress  for  various  cross-sections. 

tion  of  $s  across  the  cross-section  for  several  common 
cross-sections  of  beams.  For  the  I-beam  it  is  seen  that 
the  value  of  $s  is  nearly  constant  over  the  whole  depth  of 
the  web,  whence  the  common  practice  of  finding  the  approx- 
imate value  of  &  for  an  I-beam  by  dividing  the  shear  7  by 
the  area  of  the  web.  Fig.  13  illustrates  the  computation 
of  £8  for  a  point  in  a  cross-section  of  somewhat  complex 
shape. 

Since  the  maximum  tensile  (or  compressive)  stress  *S 
due  to  flexure  occurs  at  an  extreme  fiber  where  $a  is  zero, 
and  the  maximum  value  of  $s  occurs  at  or  near  the  neutral 
axis  where  $  is  zero  it  is  very  rarely  necessary  to  con- 
sider the  effect  of  combined  flexural  and  shearing  stresses 
at  other  points  in  the  cross-section.  For  I-beams  and 
girders  with  thin  webs  the  combined  shearing  and  flexural 


26 


MATERIALS  OF  ENGINEERING 


stresses  at  the  junction  of  web  and  flange  may,  however, 
cause  a  resulting  stress  greater  than  S  at  the  extreme  fiber. 


PROBLEM. — For  the  same  cross-section  as  is 
shown  in   Fig.   7  determine  the  shearing  unit 
10*    stress   (/SJ — horizontal  and  vertical — at  MN, 
in  terms  of  the  vertical  shear  V  at  the  section. 

s'=TbvA 


FIG.  13.  —  Determination 
of  shearing  stress  for  a  beam 
whose  cross-section  is  a  com- 
posite  figure. 


From  Fig.  7:7  =  438.3  (in.)4 

6  (at  MN)  =  1.00  +  0.43  +  0.43  +  1.00 
=  2.86  in. 


Part 
above 
MN 

Width 
(in.) 
b 

Height 
(in.) 
ft 

Area  of 
part  above 
MN 
(sq.  in.) 
A 

Distance  of 
center  of 
gravity  of 
part  from 
XX  (in.) 

V 

Static 
moment  of 
part  about 
XX 
(in.)  s 
VA 

B 

1.00 

3.00 

3.00 

3.31 

9.93 

B' 

1.00 

3.00 

3.00 

3.31 

9.93 

C 

0.43 

3.00 

0.64 

2.81 

1.80 

C' 

0.43 

3.00 

0.64 

2.81 

1.80 

D 

6.00 

1.00 

6.00 

5.31 

31.86 

Sum  of  vA's  (SyA)  55.32.     This  is  vA  of  the  formula  above. 

o    ._  v 55.32 00441F 

V  438.3X2.86  ~ 

Long  Compression  Members,  Columns,  Pillars,  Struts.— 

In  any  actual  structural  or  machine  member  nominally 
under  axial  load  there  is  always  present  bending  action 
due  to  accidental  eccentricity  of  load  and  non-homogeniety 
of  material.  In  tension  members  the  bending  action  tends 
to  decrease  under  load  and  formula  (1)  can  be  used  for  any 
length  of  member.  In  compression  members  the  bend- 
ing action  tends  to  increase  under  load,  and  for  members 
whose  length  is  more  than  four  or  five  times  their  smallest 
transverse  dimension  formula  (1)  is  not  safe  to  use.  As 
the  eccentricities  and  irregularities  which  cause  bend- 
ing action  in  compression  members  with  nominal  axial 
loading  are  necessarily  uncertain  in  amount,  formulas  for 
long  compression  members  are  necessarily  empirical. 


STRAIN  AND  STRESS 


27 


For  long  compression  members  it  is  customary  to  reduce 
P/A,  the  average  unit  stress  allowed  on  the  cross-section, 
by  an  amount  depending  on  the  length  of  the  member, 
the  manner  in  which  it  is  fastened  at  the  ends,  and  the  form 
and  size  of  the  cross-section.  The  two  column  formulas 
most  used  are  the  " straight  line"  formula,  and  the  Ran- 
kine-Gordon  formula.  The  straight  line  formula  is : 


p/A  =  S- 


(11) 


in  which  P  is  the  safe  axial  load  in  pounds;  A  is  the  area  of  the  cross- 
section  in  sq.  in.;  S  is  the  safe  unit  stress  in  compression  at  the  concave 
edge  of  the  column,  measured  in  Ib.  per  sq.  in.;  I  is  the  length  of  the 
column  in  inches;  r  is  the  least  radius  of  gyration  of  the  area  of  the 
cross-section  of  the  column,  measured  in  inches  (l/r  is  called  the  slander- 
ness  ratio  for  the  column),  and  &  is  a  constant  determined  from  tests  of 
columns  to  failure. 


The  Rankine-Gordon  formula  is 
P/A  =  . 


S 


(12) 


in  which  q  is  a  constant  determined  from  tests  of  columns  to  failure 
(not  the  same  constant  as  k  in  the  straight  line  formula)  and  the  other 
symbols  are  the  same  as  for  the  straight  line  formula. 

Values  for  the  constants  k,  q,  and  S  are  given  in  various 
engineering  handbooks  for  special  cases:  average  values 
for  general  use  are  given  in  Table  1. 

TABLE  1. — CONSTANTS  FOR  USE  WITH  COLUMN  FORMULAS' 

These  constants  are  for  working  unit  stress  in  columns.     See  Chapter  IV 
for  discussion  of  working  stress. 


Material  of  column 

S 

Pin-ended  columns 

Fixed-ended  columns1 

k 

Q 

k 

q 

Structural  steel  

15,000 
20,000 
10,000 
1,000 

70 
100 
75 
10 

0.00012 
0.00017 
0.00040 
0.00050 

40 
60 
45 
•     5 

0.000030 
0.000042 
0.000100 
0.000125 

Structural  nickel  steel  .  . 
Cast  iron  

Timber 

1  For  actual  columns  the  ends  are  never  absolutely  fixed,  and  the  general 
practice  is  to  use  values  somewhat  larger  than  those  given  in  this  column. 


28  MATERIALS  OF  ENGINEERING 

Lateral  Strain  Under  Load;  Poisson's  Ratio. — Accompanying 
tensile  or  compressive  stress  in  any  direction  there  is  a  strain  in  the 
direction  of  the  stress  and  also  a  strain  at  right  angles  to  that  direction. 
A  round  bar  placed  under  axial  tension  diminishes  slightly  in  diameter, 
and  a  round  bar  in  compression  increases  slightly  in  diameter.  Members 
with  other  shapes  of  cross-section  undergo  corresponding  changes  of 
transverse  dimensions  under  axial  load.  Within  the  proportional 
limit  for  a  member  under  axial  load  the  change  in  a  transverse  dimen- 
sion divided  by  that  dimension  is  called  the  lateral  unit  strain.  The 
ratio  of  lateral  unit  strain  produced  by  an  axial  load  to  the  axial  unit 
strain  produced  by  the  same  load  is  called  Poisson's  ratio.  Values  of 
Poisson's  ratio  for  common  materials  are: 

Steel  and  wrought  iron - 0 . 30 

Cast  iron 0. 25 

Brass 0.25 

Concrete 0. 10 

Comparatively  little  experimental  work  has  been  done  on  the  deter- 
mination of  Poisson's  ratio  for  other  materials  used  in  structures  and 
machines. 

Effect  of  Lateral  Strain  on  Strength;  Three  Theories  of  Failure  of 
Materials. — Under  the  action  of  two  stresses  at  right  angles,  or  under 
the  combined  action  of  axial  stress  and  shearing  stress  the  lateral 
strains  set  up  have  no  effect  on  the  stresses  developed,  but  they  do 
affect  the  strains  developed  at  any  point.  For  example,  a  boiler  shell 
is  subjected  to  circumferential  tensile  stress  due  to  the  bursting  action  of 
the  steam  pressure,  and  also  to  axial  stress  due  to  the  pull  of  the  boiler 
heads.  The  lateral  contraction  due  to  the  axial  stress  acts  to  diminish 
the  circumferential  stretch  due  to  bursting  pressure,  and  the  resulting 
maximum  circumferential  unit  strain  at  a  point  in  the  shell  is  less  than 
the  circumferential  unit  strain  due  to  bursting  pressure  alone.  The 
maximum  circumferential  unit  stress  is  unaffected  by  the  axial  stress. 
If  the  shell  were  subjected  to  axial  compression  from  any  source  this 
axial  compression  would  increase  the  circumferential  stretch  over  that 
due  to  bursting  pressure  alone. 

Which  action,  strain  or  stress,  produces  structural  damage  in  the 
material  of  a  member?  Or,  as  some  claim,  is  it  really  the  shearing  stress 
(on  some  oblique  plane)  which  tends  to  cause  failure  in  the  material? 
The  answer  to  these  questions  is  still  a  matter  of  debate,  and  there  are 
three  common  theories  for  the  cause  of  failure  of  material,  namely  the 
maximum  strain  theory,  the  maximum  stress  theory,  and  the  maximum 
shear  theory.  Perhaps  the  most  recent  investigations  are  those  of 
Becker  at  the  University  of  Illinois  and  those  of  Matsumura  and 
Hamabe  at  Kyoto  Imperial  University.  These  investigations  taken 
together  seem  to  indicate  that  strain  rather  than  stress  is  the  cause 


STRAIN  AND  STRESS  29 

of  failure,  but  that  shearing  stress  must  also  be  taken  into  account. 
For  a  complete  analysis  of  the  strength  of  any  structural  or  machine 
member  it  is  necessary  to  compute  both  the  maximum  unit  strain,  and 
the  maximum  unit  shearing  stress.  The  common  theory  of  strength 
of  materials  as  given  in  elementary  text-books  on  the  mechanics  of 
materials  is  the  maximum  stress  theory,  and  in  such  books  no  account 
is  taken  of  the  effect  of  lateral  strain,  or  the  "Poisson's  ratio  effect" 
as  it  is  sometimes  called,  this  is  equivalent  to  assuming  a  value  of 
zero  for  Poisson's  ratio.  For  most  cases  met  in  practice  the  common 
theory  gives  results  of  sufficient  accuracy. 

Unit  strain  is  measured  in  inches  per  inch  length,  and  is  conse- 
quently an  abstract  number,  but  it  has  become  so  fixed  a  habit  to  think 
and  write  of  strength  of  materials  in  pounds  per  square  inch  that  in 
using  the  maximum  strain  theory  the  value  of  the  unit  strain  is  usually 
multiplied  by  the  modulus  of  elasticity  of  the  material,  giving  a  value 
proportional  to  unit  strain  but  expressed  in  pounds  per  square  inch. 
This  value  is  denoted  by  the  symbol  Ed  and  in  this  text  will  be  called 
the  strain  equivalent. 

Elementary  Formulas  Involving  the  Consideration  of  Lateral  Strain.— 
If  two  stresses  act  at  right  angles  to  each  other  the  maximum  unit 
strain  is  in  the  direction  of  the  larger  stress,  and  the  magnitude  of  the 
strain  equivalent  Ed  is: 

Ed  =  Si  -  <rSi  (13) 

in  which  Si  is  the  (numerically)  larger  stress  (tensile  or  compressive) ; 
Sz  is  the  numerically  smaller  stress  (tensile  or  compressive);  and  a 
is  the  value  of  Poisson's  ratio  for  the  material.  In  using  formula  (13) 
tensile  stresses  are  considered  plus  and  compressive  stresses  are  con- 
sidered minus.  This  applies  to  formulas  (14)  and  (14a)  also. 

The  strain  equivalent  Ed'  in  the  direction  of  S?  is: 

Ed'  =  Sz-  <rSi  (13o) 

The  maximum  unit  shearing  stress  S,'  on  any  oblique  plane  is: 

S'.  =  &/2  (14) 

if  Si  and  S2  are  of  the  same  sign,  and 

5.'-^=*?  (14a) 

if  Si  and  S*  are  of  opposite  sign.  The  reason  for  the  difference  between 
formula  (14)  and  formula  (14a)  involves  analysis  in  three-dimensional 
mechanics  which  will  not  be  given  here. 

If  there  is  acting  at  a  point  in  the  cross-section  of  a  member  an  axial 
unit  stress  S  (caused  either  by  direct  axial  load  or  by  flexure)  com- 
bined with  a  shearing  unit  stress  S,  (caused  either  by  direct  shearing 
action  or  by  torsion)  the  resulting  strain  equivalent  #5  is: 


30 


MATERIALS  OF  ENGINEERING 


Formula  (15)  is  the  maximum  strain  theory  formula  corresponding 
to  the  maximum  stress  theory  formula  (8)  on  page  23. 

Under  the  combined  action  of  axial  stress  and  shearing  stress  the 
maximum  shearing  unit  stress  is  given  by  formula  (9)  for  either  the 
maximum  stress  theory  or  the  maximum  strain  theory. 

The  maximum  strain  theory  is  sometimes  called  St.  Venant's  theory; 
the  maximum  stress  theory  is  sometimes  called  Rankine's  theory; 
and  the  maximum  shear  theory  is  sometimes  called  Guest's  theory. 
As  noted  above  the  theory  commonly  used  in  engineering  computations 
is  the  maximum  stress  theory,  with  occasional  computation  of  shear- 
ing stress.  This  procedure  is  of  sufficient  accuracy  for  most  engineer- 
ing computations,  and  the  foregoing  paragraphs  on  the  maximum 
strain  theory  are  not  intended  to  disparage  the  use  of  the  common 
theory,  but  to  serve  as  an  introduction  to  the  more  exact  theory  for 
cases  where  an  unusually  high  degree  of  accuracy  is  desirable. 

Stress-strain  Diagrams  for  Materials. — The  relation  of 
stress  to  strain  for  any  material  is  conveniently  shown  by 


0.05  0.10  0.15  0.20 

Unit    Strain   Inches  per  Inch 
FIG.  14. — Typical  stress-strain  diagrams. 

a  stress-strain  diagram,  such  as  is  given  in  Figs.  14  and  15. 
Such  a  diagram  is  obtained  as  follows:  A  series  of  known 
loads  is  applied  to  a  specimen  of  the  material  by  means  of 
a  testing  machine  (see  page  267),  and  the  corresponding 
strains  measured  by  means  of  some  form  of  micrometer. 
The  unit  stresses  and  unit  strains  corresponding  respec- 
tively to  the  loads  and  strains  are  computed  and  plotted, 


STRAIN  AND  STRESS 


31 


usually  representing  unit  stresses  as  ordinates  (vertical 
distances  on  the  diagram)  and  unit  strains  as  abscissas 
(horizontal  distances) .  A  curve  drawn  through  the  plotted 
points  gives  the  stress-strain  diagram.  Figs.  14  and  15 
give  typical  stress-strain  diagrams  for  steel,  cast  iron,  and 
wood  which  are  representative  stress-strain  diagrams  for 
ductile  materials,  brittle  materials,  and  fibrous  materials, 
respectively. 


40000 


MM! 

Strain  under Sfress~ 
Permanent  Strain(5et) 
after  removal  of  Stress ' 


I  Division  =0.0002  Inches    per    Inch,  Uni-f-    5-f-nain 
FIG.  15. — Typical  stress-strain  diagrams  for  small  strains. 

Elastic  Limit,  Proportional  Limit,  Yield  Point. — In  Fig. 
15  solid  lines  indicate  strain  when  the  specimen  is  stressed 
and  broken  lines  indicate  strain  remaining  after  stress  is 
removed  (permanent  set).  In  the  stress-strain  diagrams 
obtained  for  most  materials,  it  is  seen  (broken-line  diagrams, 
Fig.  15)  that  up  to  a  certain  unit  stress  there  is  no  measur- 
able permanent  set  after  the  removal  of  load.  For  any 
material  the  lowest  unit  stress  at  which  there  can  be 
detected  permanent  set  is  called  the  elastic  limit  of  the 
material.  In  Fig.  15  the  elastic  limits  are  shown  at  E. 

For  most  material  up  to  a  certain  unit  stress  the  stress- 
strain  diagram  does  not  deviate  appreciably  from  a  straight 
line,  and  Hooke's  law  holds.  For  any  material  the  lowest 
unit  stress  at  which  there  can  be  detected  a  deviation 
from  Hooke's  law  is  called  the  limit  of  proportionality  of 
stress  to  strain,  or  more  briefly,  the  proportional  limit. 
In  Fig.  15  proportional  limits  are  denoted  by  P. 

For  ductile  materials,  such  as  mild  steel,  soft  brass,  etc., 


32  MATERIALS  OF  ENGINEERING 

especially  for  rolled  or  forged  materials,  the  stress-strain 
diagram  frequently  shows  a  sharp  break  as  at  F  (Figs.  14 
and  15).  If  in  making  a  test  of  such  material  the  load  is 
applied  continuously  the  scale  beam  of  the  testing  machine 
" drops"  at  a  stress  corresponding  to  the  sharp  break  in 
the  stress-strain  diagram  and  strain  can  be  detected  by 
direct  measurement  with  a  pair  of  dividers.  The  unit 
stress  (Y,  Fig.  15)  at  which  a  very  sudden  change  takes 
place  in  the  strain  is  called  the  yield  point.  In  general, 
only  ductile  materials  show  a  yield  point.  In  commercial 
testing  the  yield  point  is  sometimes  erroneously  called  the 
elastic  limit. 

Ultimate  Strength. — In  tension  tests  or  shear  tests  of 
materials,  and  in  compression  tests  of  non-ductile  mate- 
rials there  is  found  a  well-defined  load  which  is  the  maximum 
carried  before  rupture  occurs.  The  unit  stress  correspond- 
ing to  this  load  is  called  the  ultimate  strength  or,  more  briefly 
the  ultimate  for  the  material. 

Significance  of  the  Elastic  Limit,  the  Proportional  Limit 
and  the  Yield  Point. — For  any  material  the  values  deter- 
mined for  the  elastic  limit  and  the  proportional  limit  are 
dependent  upon  the  precision  of  the  measuring  instruments 
and  methods  used  in  their  determination.  Tests  are  rarely 
made  to  determine  the  elastic  limit,  since  such  tests  in- 
volve repeated  application  and  release  of  stress  and  take 
a  very  long  time.  The  elastic  limit  of  a  material  is  an 
indication  of  the  stress  which  the  material  will  withstand 
without  appreciable  permanent  distortion.  This  is  of 
value  in  the  study  of  materials  for  machine  tools  in  which 
any  appreciable  permanent  distortion  would  mean  very 
serious  damage  to  the  machine. 

The  elastic  limit  has  been  defined  by  some  writers  on 
Mechanics  of  Materials,  to  be  the  unit  stress  below  which 
any  material  was  perfectly  elastic,  and  below  which  the 
material  would  be  able  to  withstand  without  rupture  an 
infinite  number  of  repetitions  of  stress.  Whether  in  a 
test  under  static  load — such  a  test  as  is  made  in  a  testing 
machine — with  instruments  of  higher  precision  than  those 


STRAIN  AND  STRESS  33 

now  available  there  could  be  found  an  absolute  elastic  limit 
for  any  actual  material  is  doubtful.  Under  repeated 
stresses  rupture  has  occurred  at  computed  unit  stresses 
considerably  lower  than  the  proportional  limit  as  deter- 
mined in  the  ordinary  laboratory  test. 

Some  physicists  and  some  testing  engineers  maintain 
that  for  most  materials  the  elastic  limit  and  the  propor- 
tional limit  occur  at  identically  the  same  stress;  others 
dispute  this  statement;  while  still  others  claim  that,  if 
instruments  of  sufficient  precision  were  available,  for  act- 
ual materials  some  extremely  minute  permanent  set  and 
some  very  slight  deviation  from  Hooke's  law  would  be  found 
for  any  stress,  however,  small.  However,  for  practical 
purposes  the  two  limits  are  identical  and  they  will  be  so 
regarded  in  this  book,  and  the  term  proportional  limit 
commonly  used.  Practically  the  proportional  limit  of  a 
material  is  the  criterion  of  its  elastic  strength  under  static 
loading. 

As  suggested  above  the  exact  location  of  the  proportional 
limit  depends  on  the  precision  of  the  instruments  used 
in  measuring  loads  and  strains,  and  on  the  accuracy  of  plot- 
ting the  stress-strain  diagram.  Following  the  practice  re- 
commended by  the  Committee  on  Standard  Methods  of 
Testing,  appointed  by  the  American  Society  for  Testing 
Materials,  the  proportional  limit  will  be  located  at  that 
stress  for  which  the  stress-strain  diagram  shows  the  first 
deviation  from  a  straight  line,  the  measurements  of  strain 
being  made  with  a  micrometer  reading  to  0.0001  in. 

The  yield  point  indicates  a  sudden  change  from  a  condi- 
tion of  nearly  perfect  elasticity  to  one  of  a  high  degree  of 
plasticity.  Below  the  yield  point  Hooke's  law  is  a  very 
close  approximation  to  the  truth,  and  permanent  sets  are 
very  small.  If  the  material  of  a  structure  or  a  machine  is 
stressed  beyond  the  yield  point,  the  resulting  distortion  is 
so  great  that  the  usefulness  of  the  structure  or  machine  is 
usually  at  an  end,  unless  the  overstressed  area  is  very  small. 
For  ductile  materials,  the  yield  point  should,  in  general, 
be  regarded  as  the  practical  ultimate  strength  under  static 


34  MATERIALS  OF  ENGINEERING 

loads  for  tension  as  well  as  for  compression.  Brittle 
materials  have  no  yield  point,  and  the  ultimate  strength 
is  probably  the  most  reliable  limit  of  strength  for  static 
loads.  The  yield  point  is  much  more  readily  determined 
than  is  the  proportional  limit,  and  specifications  for  rolled 
materials  usually  contain  requirements  for  the  yield  point 
rather  than  for  the  proportional  limit. 

Behavior  of  Materials  in  a  Partially  Plastic  State.— 
In  most  cases  the  material  of  structures  and  machines  is 
subjected  to  stresses  so  low  that  the  action  is  almost  per- 
fectly elastic.  The  behavior  of  material  under  stresses 
so  high  that  plastic  action  is  set  up  is  of  interest  for  two 
reasons:  (1)  In  the  process  of  shaping  and  fabricating  the 
material  it  is  not  infrequently  necessary  to  bend,  to  stretch, 
or  to  punch  the  material  cold;  (2)  the  behavior  of  the 
material  under  accidental  overloads  is  frequently  of  im- 
portance, and  for  some  members,  such  as  ball  and  roller 
bearings,  the  stresses  at  points  of  contact  are  above  the 
proportional  limit. 

In  fabricating  members  of  steel  structures,  such  as  beams 
and  columns,  it  is  frequently  necessary  to  bend  them  to 
shape  or  to  punch  holes  in  them.  Those  processes  involve 
local  stresses  beyond  the  yield  point  of  the  material. 
Material  which  possesses  high  ductility  is  not  seriously 
injured  by  such  treatment,  but  brittle  material  under 
such  conditions  would  be  shattered. 

In  selecting  the  material  to  be  used  for  a  structure  or 
a  machine,  it  is  frequently  of  importance  to  consider  the 
effects  of  accidental  overload.  An  excellent  example  is 
furnished  in  the  selection  of  material  for  parts  of  railway 
rolling  stock,  such  as  car  couplers,  draft  rigging,  side  frames, 
etc.  For  such  parts  it  is  evident  that  a  tough  material 
which,  even  after  it  is  badly  distorted,  still  possesses  con- 
siderable strength  is  preferable  to  a  brittle  material  which, 
though  it  may  stand  a  higher  unit  stress  before  rupture, 
snaps  suddenly  with  very  little  strain  or  other  warning  of 
approaching  failure,  if  rupture  does  occur.  A  material 
which  after  severe  distortion  still  possesses  high  strength 


STRAIN  AND  STRESS 


35 


is  called  "  tough."  Fig.  16  gives  stress-strain  diagrams 
for  a  tough  material,  and  for  two  others  less  tough.  The 
toughness  of  a  material  may  be  measured  by  the  area 
under  the  complete  stress-strain  diagram. 

The  desirability  of  insuring  the  parts  of  railway  cars 
against  sudden,  shattering  failure  has  caused  the  very 
general  replacement  of  cast-iron  parts  by  steel  castings. 
For  a  similar  reason  cast-iron  columns  for  buildings  have 
been  generally  discarded  in  the  best  practice.  It  is  a 
general  rule  in  machine  design  not  to  use  a  brittle  material 
in  direct  tension  if  it  can  be  avoided. 


Structual  Steel  /5  Toughesh 
Hard  Steel  is  Strongest  i 
Soft  Iron  is  most  Ductile  \ 


Unit   Strain 
FIG.  16. — Stress-strain  diagrams  showing  different  degrees  of  toughness. 

In  parts  of  machines  which  involve  the  carrying  of  heavy 
loads  on  spherical  or  cylindrical  surfaces — such  as  ball 
bearings,  roller  bearings,  car  and  wagon  wheels,  chain  links 
— there  are  set  up  in,  in  service,  stresses  beyond  the  propor- 
tional limit  of  the  material.  Such  parts  do  not  last 
indefinitely;  they  wear  out,  and  their  length  of  life  is  de- 
pendent on  the  properties  of  the  material  when  stressed 
beyond  the  proportional  limit.  In  wire  rope,  band  saws, 
and  other  flexible  machine  parts  which,  in  use,  are  re- 
peatedly bent  as  they  pass  round  pulleys  and  sheaves,  the 
yield  point  of  the  material  is  frequently  exceeded,  and 
such  members  show  considerable  permanent  distortion 
after  a  short  time  in  service,  and  finally  wear  out. 

Effect  of  Stress  Beyond  the  Yield  Point.— In  1881, 
Johann  Bauschinger  of  Munich  published  the  results  of  a 


36 


MATERIALS  OF  ENGINEERING 


long  series  of  experiments  which  demonstrated  that,  if  a 
ductile  material  was  stressed  in  one  direction  beyond 'its 
yield  point,  for  subsequent  stress  in  the  same  direction,  the 
yield  point  and  the  proportional  limit  were  raised,  that  for 
subsequent  stresses  in  the  reverse  direction  the  yield  point 
and  the  proportional  limit  were  lowered,  and  that  in  any 
case  the  toughness  of  the  material  was  diminished.  Fig.  17 
illustrates  the  properties  of  material  stressed  beyond  the 
yield  point.  Bauschinger  also  showed  that  time  was  nec- 
essary for  the  particles  of  the  material  to  adjust  themselves 
after  overstress,  and  that  for  subsequent  stresses  in  either 


f}=  Proportional  Limit,  Is*  Stress 


yield  Point.  /s.r  Stress 
„     2nd     » 

n  //3V       „ 


FIG.  17. — Stress-strain  diagram  showing  effect  of  repetition  of  stress  beyond  the 

proportional  limit. 

direction  the  yield  point  and  the  limit  of  proportionality 
of  overstressed  materials  were  raised  by  rest. 

The  properties  of  cold-rolled  steel  and  of  cold-drawn 
wire  are  explained  by  Bauschinger 's  tests,  as  is  the  "  springi- 
ness" of  hammered  steel  or  brass  plates.  In  the  process 
of  cold-drawing  or  cold-rolling  the  material  is  stretched 
well  beyond  its  yield  point.  Steel,  iron,  copper,  brass, 
aluminum  and  zinc  are  rendered  stronger  by  cold-rolling, 
cold-drawing  or  cold-hammering.  The  cold-working  of 
metal  generally  decreases  its  ductility  and  its  toughness. 
To  a  large  degree  heating  followed  by  slow  cooling  (anneal- 


STRAIN  AND  STRESS  37 

ing)  removes  the  effects — beneficial  and  injurious  alike — 
of  cold-working.  The  resistance  of  metal  to  repeated 
stress  is  probably  not  increased  by  cold-working. 

Resistance  of  Materials  to  Impact. — In  selecting  mate- 
rials for  members  which  must  resist  impact,  it  must  be 
borne  in  mind  that  there  are  two  factors  to  be  considered: 
total  stress  allowable  in  the  member,  and  total  strain  allow- 
able. Resistance  to  impact  is  a  function  of  both  these  fac- 
tors. In  this  connection  it  may  be  noted  that  "the  force 
of  a  blow"  can  not  be  computed  unless  not  only  the  energy 
of  the  blow  is  known,  but  also  the  rigidity  of  the  body 
striking  the  blow  and  of  the  parts  on  which  the  blow  is 
delivered,  for  example,  a  piece  of  iron  weighing  100  Ib.  fall- 
ing from  a  height  of  20  ft.  would  deliver  2,000  ft.-lb.  of 
energy  when  it  struck  a  body,  but  the  force  set  up  if  it 
struck  soft  earth  would  be  much  less  than  the  force  set  up 
if  it  struck  a  rigid  concrete  foundation. 

For  materials  which  must  withstand  heavy  accidental 
impact  without  actual  rupture,  toughness  is  the  prime 
requisite.  Toughness  has  been  defined  on  page  3  and  it 
may  be  noted  here  that  the  toughness  of  a  material  may  be 
measured  by  the  area  under  the  stress-strain  diagram  for 
that  material  (see  Fig.  14).  A  striking  illustration  of  the 
resistance  of  materials  to  rupture  under  impact  is  furnished 
by  comparing  the  action  of  oak  with  that  of  cast  iron. 
Under  static  load  cast  iron  is  about  three  times  as  strong 
as  oak,  but  the  strain  which  oak  will  stand  before  it  is  rup- 
tured is  about  nine  times  the  strain  which  cast  iron  will 
stand.  The  area  under  the  stress-strain  diagram  for  oak 
is  about  three  times  that  for  cast  iron,  and  under  impact 
loads  oak  is  about  three  times  as  strong  as  cast  iron. 

The  ability  of  a  material  to  resist  impact  without  per- 
manent distortion  is  measured  by  the  area  under  the  stress- 
strain  diagram  up  to  the  elastic  limit  (for  practical  purposes 
up  to  the  proportional  limit).  If  elastic  resistance  to  im- 
pact is  desired  a  material  with  a  high  proportional  limit 
or  a  low  modulus  of  elasticity  should  be  used.  A  good 
illustration  of  the  difference  between  elastic  resistance  to 


38  MATERIALS  OF  ENGINEERING 

impact  and  resistance  to  rupture  under  impact  is  furnished 
by  a  comparison  of  the  action  of  high-carbon  steel  with  that 
of  low-carbon  steel.  High-carbon  steel  has  the  higher  pro- 
portional limit  and  has  a  greater  area  under  its  stress-strain 
diagram  up  to  the  proportional  limit.  It  is  superior  to 
low-carbon  steel  in  its  elastic  resistance  to  impact  and  is 
used  for  such  members  as  springs.  Low-carbon  steel, 
howeverj  has  a  greater  area  under  the  whole  of  its  stress- 
strain  diagram  than  has  high-carbon  steel  (see  Fig.  16),  on 
account  of  its  much  greater  ductility,  and  low-carbon  steel 
offers  greater  resistance  to  rupture  under  impact  than  does 
high-carbon  steel,  and  is  used  for  such  members  as  chains 
and  car  couplers  in  which  ability  to  withstand  occasional 
heavy  shock  without  rupture  is  of  great  importance. 
Wrought  iron  has  a  low  elastic  resistance  to  impact,  but, 
like  low-carbon  steel  has  a  high  resistance  to  rupture  under 
impact.  In  this  respect  it  does  not  show  any  marked 
superiority  to  the  better  grades  of  low-carbon  steel,  but  is 
superior  to  the  poorer  grades. 

In  considering  resistance  to  static  load,  it  is,  in  general, 
necessary  to  consider  only  the  unit  stresses  set  up  in  the 
most  stressed  fibers  of  a  member — in  the  smallest  cross- 
section  of  a  rod  in  tension,  for  example.  If  a  machine  or 
a  structure  is  to  withstand  impact,  the  total  deformation  of 
any  part  is  effective  in  increasing  the  resistance  to  impact. 
Two  rods  of  the  same  material  with  equal  cross-section 
have  the  same  static  strength  in  tension  irrespective  of 
their  relative  lengths;  but  their  ability  to  withstand  impact 
varies  directly  as  their  length. 

Stiffness,  Significance  of  the  Modulus  of  Elasticity.— 
The  modulus  of  elasticity  of  a  material  is  an  index  of  its 
stiffness  or  rigidity  and  is  the  ratio  of  unit  stress  within 
the  proportional  limit  to  the  corresponding  unit  strain. 
The  stiffness  or  rigidity  of  a  strong  material  under  working 
loads  may  be  no  higher  than  the  rigidity  of  a  weaker 
material.  The  best  illustration  of  the  distinction  between 
strength  and  stiffness  is  found  in  the  action  of  steel.  All 
grades  of  steel  from  the  softest  rivet  steel  to  the  hardest 


STRAIN  AND  STRESS 


39 


tool  steel  have  about  the  same  modulus  of  elasticity — 
30,000,000  Ib.  per  square  inch.  This  is  illustrated  in  Fig.,  18 
which  shows  typical  stress-strain  diagrams  for  various 
grades  of  steel.  In  Fig.  18  the  slope  of  the  line  OA  indi- 
cates the  value  of  the  modulus  of  elasticity  for  all  the  steel 
specimens.  If  the  unit  stresses  actually  set  up  in  a  ma- 
chine member  or  structural  part  are  within  the  limit  of 


200.000 


0001   0002   0.003  0.004  0.005  0006  0.007  0.006  0.009 
Unit  Strain  Inches  per  Inch 

FIG.  18. — Stress-strain  diagrams  for  various  grades  of  iron  and  steel. 

proportionality,  it  makes  no  difference  in  the  rigidity 
whether  soft  steel  or  hard  steel  is  used.  In  machine  tools 
it  has  been  sometimes  proposed  to  remedy  too  great  de- 
flection by  replacing  the  flexible  parts  with  others  made  of 
a  stronger,  harder  steel;  this  usually  does  no  good,  as  the 
stresses  in  machine  tool  parts  are  usually  low,  and  it  is 
the  modulus  of  elasticity  of  the  steel  rather  than  the 
strength  which  counts.  The  use  of  high-strength  nickel 
steel  for  long-span  bridges  will  allow  the  use  of  smaller 


40  MATERIALS  OF  ENGINEERING 

structural  m  >mbers,  but  the  stiffness  of  the  bridge  will  be 
somewhat  diminished  by  the  use  of  these  smaller  members. 
Coefficient  of  Expansion;  Stresses  due  to  Tempera- 
ture.— The  application  of  heat  expands  all  solids.  The 
amount  of  expansion  per  inch  of  length  per  degree  of 
rise  of  temperature  in  an  unrestrained  piece  of  material 
is  called  the  coefficient  of  expansion  for  the  material.  In 
Table  2  are  given  average  values  for  the  coefficient  of 
expansion  of  some  of  the  common  materials  based  on  the 
Fahrenheit  scale  of  temperature. 

TABLE  2. — COEFFICIENTS  OF  EXPANSION 

The  value  given  is  for  the  "linear"  coefficient  of  expansion;  that  is,  is  the 
expansion  per  inch  length  per  degree  (Fahr.)  rise  of  temperature. 

Coefficient  of 

expansion 
Material  (Fahrenheit) 

Aluminum 0. 0000130 

Brass ...A.!  I     0.0000101 

Bronze, 0. 0000098 

Copper 0. 0000089 

Cast  iron 0. 0000056 

Wrought  iron 0 . 0000065 

Steel 0.0000066 

Zinc.. 0.0000141 

Brick 0.0000031 

Concrete 0. 0000079 

from..  0.0000031 


Marbl°    (to ., 0.0000079 

Plaster  (white) 0. 0000092 

Porcelain 0 . 0000020 

Slate 0. 0000058 

Wood. 0. 0000028 

Brick  masonry 0 . 0000040 

If  a  machine  or  structural  member  is  restrained  from 
free  expansion  under  application  of  heat  there  is  set  up  a 
stress  in  the  member.  If  x'  is  the  expansion  which  would 
take  place  if  the  member  were  unrestrained  and  x  is  the 
expansion  which  actually  does  take  place  due  to  rise  of 
temperature  the  unit  stress  S  set  up  is : 

«      x/-xv 
S      -j-E 

in  which  I  is  the  length  of  the  member  and  E  is  the  modulus  of  elasticity 
of  the  material. 


STRAIN  AND  STRESS  41 

Selected  References  for  Further  Study 

TEXTS  ON  THE  MECHANICS  OF  MATERIALS 

1.  Elementary  texts  which  do  not  require  a  knowledge  of  calculus: 

MURDOCK:  "Strength  of  Materials,"  New  York,  1911. 
MERRIMAN:  "Strength   of    Materials"    (to   be   distinguished   from 

"Mechanics  of  Materials"  by  the  same  author),  New  York, 

1912. 
SLOCUM:  "Resistance  of  Materials,"  Boston,  1914. 

2.  Elementary  texts  which  require  a  knowledge  of  the  calculus: 

BOYD:  "Strength  of  Materials,"  New  York,  1917. 
ANDREWS:  "The  Strength  of  Materials,"  New  York,  1916. 
FULLER  AND  JOHNSON:  "Applied  Mechanics,"  Vol.  II,  New  York, 

1919. 
HOUGHTON:  "The  Elements  of  Mechanics  of  Materials,"  New  York, 

1909. 
SLOCUM  AND  HANCOCK:  "Text-book  on  the  Strength  of  Materials," 

Boston,  1906. 

3.  More  comprehensive  texts: 

MERRIMAN:  "Mechanics  of  Materials,"  New  York,  1915. 
MORLEY:  "Strength  of  Materials,"  London,  1913. 
CHURCH:  "Mechanics  of  Engineering,"  New  York,  1908. 
BURR:  "The  Elasticity  and  Resistance  of  the  Materials  of  Engineer- 
ing," New  York,  1915. 
LANZA:  "Applied  Mechanics,"  New  York,  1910. 

4.  Special  references: 

ANDREWS:  "The  Strength  of  Materials,"  New  York,  1908.     Chap. 

VI.  gives  a  discussion  of  graphical  methods  for  determining 

moment  of  inertia. 
WATERBURY:  "Stresses  in  Structural  Steel  Angles,"  New  York,  1917, 

and  L.  J.  Johnson,  Trans.  Am.  Soc.  C.  E.,  Vol.  LVI,  p.  169 

(1906),  give  discussions  of  stresses  in  beams  when  the  loading  is 

not  parallel  to  a  "principal  axis"  of  the  cross-section  of  the 

beam. 
BOYD:  "Strength  of  Materials,"  New  York,   1917,  Chap.  XVIII, 

gives  a  discussion  of  curved  beams  and  hooks. 
LANZA:  "Applied    Mechanics,"    New    York,    1910,   Chap.   X  gives 

a  discussion  of  the  strains  and  stresses  in  bodies,  taking  lateral 

strain  into  account.     A  knowledge  of  calculus  is  necessary  to 

enable  a  student  to  follow  the  discussion. 
LOVE:  "A  Treatise  on  the   Mathematical   Theory  of  Elasticity," 

Cambridge   (England),    1906,  is  a  very  elaborate  treatise  on 

strain  and  stress.     A  knowledge  of  advanced  mathematics  is 

required. 


CHAPTER  III 

THE  RESISTANCE  OF  MATERIALS  TO  REPEATED 

STRESS 

Importance  of  Resistance  to  Repeated  Stress. — The 
strength  of  materials  is  commonly  determined  by  tests 
under  a  load  gradually  increasing  from  zero  to  the  ultimate 
of  the  test  specimen.  In  nearly  all  computations  of  stress 
and  strain  the  basis  of  the  computation  is  the  action  pro- 
duced by  a  steady  load  applied  but  once.  For  more  than 
half  a  century  it  has  been  recognized  that  under  loads 
repeated  many  thousands  of  times  the  behavior  of  material 
might  be  quite  different  from  the  behavior  under  a  single 
application  of  load.  The  growing  use  of  high-speed 
machinery  has  been  especially  influential  in  necessitating 
the  consideration  of  the  strength  of  material  under  repeated 
stress. 

In  a  general  way  the  difference  in  the  behavior  of  material 
under  repeated  loads  and  under  a  single  load  (or  a  load 
applied  but  a  few  times)  is  shown  by  the  tendency  toward 
gradual  breakdown  of  the  material  under  repeated  load. 
Under  a  single  application  of  load  the  material  of  a  struc- 
ture either  withstands  the  load  or  it  fails;  under  load 
repeated  many  thousands  of  times  the  material  may  with- 
stand the  load  for  awhile,  and  then  fail  by  the  gradual 
spread  of  cracks  or  other  local  injuries  to  the  material. 
Under'  repeated  load  local  strains  which  would  be  of  no 
importance  if  but  one  loading  were  to  be  applied  may  form 
a  nucleus  for  damage  which  gradually  spreads  until  the 
whole  member  fails. 

Loss  of  Energy  during  Application  and  Release  of  Load. 
If  stress  is  applied  to  any  member  of  a  machine  or  a 
structure  and  then  is  released,  that  member  is  said  to  have 
been  carried  through  a  cycle  of  stress.  If  the  action  of  the 

42 


REPEATED  STRESS 


43 


member  were  perfectly  elastic  the  stress-strain  diagram  for 
such  a  cycle  would  be  a  straight  line  as  shown  at  OA,  Fig 
1.9.  If  there  is  inelastic  action  the  stress-strain  diagram 
for  the  cycle  of  stress  will  not  be  a  straight  line,  but  will 
enclose  a  small  area  as  shown  at  O'A',  Fig.  19.  The  area 
enclosed  within  the  stress-strain  diagram  for  a  cycle  of 
stress  represents  energy  lost,  and  this  lost  energy  is  called 
mechanical  hysteresis. 

The  minute  amount  of  energy  lost  during  a  cycle  of  stress 
is,  presumably,  transformed  into  heat,  and  this  heat  is 
accompanied  by  microscopic  wear  on  the  small  particles 


Strain 


FIG.  19. — Stress-strain  diagrams  for  one  cycle  of  stress.     OA,  No  appreciable 
inelastic  action;  O'A',  inelastic  action  evident. 

of  the  material  as  they  slide  over  each  other.  If  the  cycle 
of  stress  is  repeated  a  great  many  times  the  cumulative 
effect  of  this  wearing  action  so  weakens  the  material  that 
small  cracks  begin  to  form;  these  cracks  spread  and  finally 
cause  the  material  to  rupture.  The  growth  of  microscopic 
cracks,  or  "slip  lines,"  in  metals  was  very  beautifully 
shown  in  1899  by  Ewing  and  Rosenhain.  Fig.  20  shows 
the  development  of  these  "slip  lines,"  as  the  microscopic 
flaws  were  called  by  their  discoverers,  for  a  test  piece  of 
pure  iron  subjected  to  repeated  stress  of  considerable 
magnitude  (well  beyond  the  yield  point).  The  first  slip 
lines  appear  between  particles  of  metal  within  a  crystal 
as  these  particles  slide  one  on  another;  as  further  repeti- 


44 


MATERIALS  OF  ENGINEERING 


REPEATED  STRESS  45 

tions  of  stress  occur  larger  cracks  open  and  finally,  cause 
rupture. 

The  view  of  failure  of  metals  under  repeated  stress  for- 
merly common  was  that  under  repeated  stress  metal 
" crystallized,"  and  that  failure  took  place  at  the  junction 
of  the  crystals  formed.  This  view  has  been  generally 
discarded  by  metallurgists;  all  metals  are  crystalline  in 
structure  under  the  first  stress  as  well  as  after  many  stresses ; 
moreover,  as  shown  by  the  experiments  of  Ewing  and 
Rosenhain>  the  first  evidences  of  failure  are  usually  seen 
not  at  the  junction  of  crystals,  but  within  the  crystals. 
The  theory  of  the  gradual  development  of  microscopic 
cracks — the  " micro-flaw"  theory — is  generally  held  today 
rather  than  the  "crystallization"  theory. 

Mechanical  Hysteresis  at  Low  Stresses. — If  the  elastic 
limit  of  a  material  as  determined  by  ordinary  static  tests 
were  an  absolute  elastic  limit,  there  would  be  no  loss  of 
energy  and  no  wear  during  cycle  of  stress  within  this  limit 
and  the  material  might  be  expected  to  withstand  an  in- 
finite number  of  repetitions  of  stress  without  failure.  For 
actual  materials  this  is  not  true.  The  elastic  limit  deter- 
mined in  static  test  depends  on  the  precision  of  the  appara- 
tus used  in  determining  it;  moreover,  actual  material  is 
never  perfectly  homogeneous,  nor  are  the  members  of  any 
actual  machine  or  structure  free  from  high  localized  stresses. 
While  these  local  stresses  might  not  appreciably  affect  the 
static  strength  of  the  machine  or  structure,  under  repeated 
loads  these  high  local  stresses  may  start  cracks  which, 
spreading,  may  ultimately  cause  failure. 

Localized  Stress  in  Structural  and  Machine  Members.— 
Localized  stress  in  structural  and  machine  parts  may  be 
caused  either  by  external  irregularities  of  outline,  uneven 
application  of  load,  or  internal  non-homogeniety.  Such 
localized  stresses  are  neglected  in  the  ordinary  formulas 
of  mechanics  of  materials,  such  as  are  given  in  Chapter 
II.,  and  may  reach  magnitudes  several  times  those  of  the 
computed  unit  stresses.  Sudden  changes  in  cross-section, 
sharp  corners,  especially  inward-projecting  corners,  and 


46  MATERIALS  OF  ENGINEERING 

rough  surface  finish  are  among  the  common  external 
factors  which  cause  high  localized  stress.  Sharp  corners 
of  bearing  blocks,  and  poor  fit  of  pin  bearings  are  among  the 
common  factors  causing  high  localized  stress  at  points  of 
contact  of  adjacent  structural  or  machine  parts.  Small 
particles  of  slag  or  of  oxidized  iron,  and  imperfectly,  joined 
crystals  are  among  the  common  internal  irregularities 
of  structure  which  cause  high  localized  stress.  While 
these  high  localized  stresses  are  not  given  by  the  common 
formulas  of  mechanics  of  materials  yet  when  the  ordinary 
computed  stresses  are  increased,  in  general  these  localized 
stresses  also  increase,  so  that  their  destructive  effect  may  be 
lessened  by  reducing  the  computed  stress  on  the  member. 
As  has  been  noted  in  a  preceding  paragraph  these  localized 
stresses  act  over  such  minute  areas  that  they  affect  the 
static  strength  of  the  member  but  little.  They  are  not 
negligible,  however,  in  their  effect  on  the  strength  of  mem- 
bers subjected  to  repeated  stress.  High  localized  stress 
may  cause  a  crack  to  start  either  directly  or  by  cold-work- 
ing the  metal  (see  p.  36)  until  it  becomes  brittle  where  the 
localized  stress  exists.  This  crack  itself  extends  the  dis- 
continuity of  the  metal,  and  at  its  root  the  localized-stress 
effect  is  still  further  increased.  As  the  crack  extends  the 
effect  on  the  piece  is  as  if  a  minute  saw  cut  were  being  made 
further  and  further  into  the  piece,  until  the  member  under 
the  repeated  load  snaps  short  off.  Not  every  localized 
stress  develops  a  crack  to  failure  of  the  piece,  but  every 
high  localized  stress  is  a.  potential  source  of  progressive 
structural  failure.  Homogeneity  of  internal  structure, 
smoothness  of  internal  surface,  and  avoidance  of  sharp 
corners  and  sudden  changes  of  cross-section  may  be  more 
important  in  the  design  of  machine  parts  than  is  high  static 
strength  of  material. 

Shoulders  of  crankshafts  and  of  axles,  corners  of  key- 
ways  in  shafts,  the  root  of  screw  threads,  and  rivet  holes 
are  examples-  of  locations  where  high  localized  stress  is 
likely  to  occur. 

Both   in  laboratory  tests  and  in  actual  service  (e.g., 


REPEATED  STRESS 


47 


torpedo-boat  shafts  and  car  axles)  materials  have  failed 
under  computed  repeated  stresses  lower  than  the  elastic 
limit  as  determined  by  static  tests. 

Repeated  Stress  Tests. — There  is  at  the  present  time  no 
entirely  satisfactory  standard  for  measuring  the  resistance 
of  materials  to  repeated  stress.  The  standard  most  nearly 
satisfactory  is  found  in  the  result  of  laboratory  tests  to 
failure  of  materials  under  repeated  stress,  supplemented 
by  a  study  of  the  successes  and  failures  of  materials  sub- 
jected to  repeated  stress  in  practice.  The  earliest  and 
the  most  extensive  series  of  repeated  stress  tests  is  the  series 


0           1000000      2000000        3000000    -WOOOOO^  K)3        5   10*        5    I05         b  Vf         b  I07 

Number  of  Repetitions    of  Stress  (N)  Number  of  Repetitions    of  Stress (N) 

Necessary    to    Cause    Failure  Necessary    to   Cause   Failure 

(a)  (b) 

FIG.  21. — Typical  diagrams  of  results  of  repeated  stress  tests.  Diagram  (a) 
drawn  to  ordinary  coordinates.  Diagram  (6)  gives  same  test  results  as  diagram 
(a)  drawn  to  logarithmic  coordinates. 

made  by  Wohler  for  the  Prussian  Government  from  1858 
to  1870.  Many  other  investigators  have  made  tests,  none, 
however,  making  tests  so  extensive  as  those  of  Wohler. 
Fig.  21  shows  the  results  of  several  typical  series  of  tests 
on  materials  under  repeated  stress.1  In  this  figure,  plotted 
with  unit  stress  as  ordinates  and  number  of  repetitions  to 
cause  failure  as  abscissas,  it  is  seen  that  the  curves  have  a 
steep  downward  slope  at  the  beginning,  and  become  nearly 
horizontal  for  about  3,000,000  repetitions  of  stress.  By 
earlier  writers  on  mechanics  of  materials  it  was  assumed 

1  See  p.  284  for  description  of  testing  machines  for  testing  of  the  strength 
of  materials  under  repeated  stress. 


48  MATERIALS  OF  ENGINEERING 

that  the  curves  became  so  nearly  horizontal  after  a  few 
million  repetitions  that  the  corresponding  unit  stress 
might  be  assumed  as  the  strength  of  the  material  under  an 
infinite  number  of  repetitions  of  stress.  The  unit  stress 
corresponding  to  the  ordinate  of  the  curve  after  it  had  be- 
come horizontal  was  called  the  endurance  limit  of  the  ma- 
terial. In  many  cases,  the  endurance  limit  as  thus  found 
for  a  material  is  roughly  proportionel  to  the  ultimate  static 
strength  though  there  are  exceptions  to  this,  especially 
for  cold-worked  metal.  If  material  was  subjected  to  repe- 
tition of  stress  varying  from  zero  to  a  maximum  the  en- 
durance limit,  was  found  to  be  not  far  from  one-half  the 
ultimate  strength.  If  the  material  was  subjected  to  stress 
varying  from  a  maximum  in  one  direction  to  an  equal 
stress  in  the  opposite  direction  (complete  reversal  of  stress) 
the  endurance  limit  was  found  to  be  from  one-quarter  to 
one-third  the  ultimate  strength.  Based  on  the  researches 
of  Weyrauch  and  Launhardt,  the  following  formula  was 
proposed  by  J.  B.  Johnson: 

Q 


Se  =  the  endurance  limit  for  the  material. 

Su  =  the  (static)  ultimate  of  the  material. 

Q  =  the  ratio  of  minimum  stress  to  maximum  stress  during  a  cycle  of 
stress  (for  stress  varying  from  zero  to  a  maximum,  Q  =  0;  for  completely 
reversed  stress,  Q  =  —  1). 

The  use  of  the  endurance  limit  gives  fairly  reliable  re- 
sults for  members  which  will  not  have  to  withstand  more 
than  a  few  million  repititions  of  stress,  but  in  view  of  the 
fact  that  the  most  extensive  laboratory  tests  have  been 
carried  only  to  a  few  million  repititions,  and  that  in  service 
such  members  as  shafts,  car  axles,  and  piston  rods  are  fre- 
quently required  to  withstand  several  hundreds  of  millions 
of  repetitions  of  stress  before  wearing  out,  it  seems  doubtful 
whether  the  endurance  limit  as  determined  above  is  entirely 
reliable  for  such  members. 

In  1910  O.  H.  Basquin  of  Northwestern  University 
pointed  out  that  an  examination  of  the  results  of  numerous 


REPEATED  STRESS  49 

series  of  repeated  stress  tests  indicated  that  for  nearly  all 
the  range  covered  by  such  tests  the  law  of  resistance  to 
repeated  stress  might  be  expressed  by  the  equation 

S  =  KNm  (2) 

in  which  S  =  the  maximum  unit  stress  developed  in  the  test  piece. 

N  =  the  number  of  repetitions  of  stress  necessary  to  cause  failure. 
K  and  m  are  constants  depending  on  the  material  and  somewhat 
on  the  manner  of  making  the  test. 

This  is  known  as  the  exponential  equation  for  repeated 
stress.  Another  form  of  expression  for  the  above  equation, 
frequently  more  convenient  is 

log  S  =  log  K  +  m  log  N  (3) 

If  the  logarithms  of  S  and  N  are  plotted,  or  if  values 
of  S  and  N  are  plotted  on  logarithmic  cross-section  paper 
equation  (3)  is  represented  by  a  straight  line.  Fig.  216 
shows  the  results  of  the  same  series  of  tests  as  is  given  in 
Fig.  2 la  but  in  Fig.  216  the  coordinates  are  logarithmic. 
For  large  values  of  N  the  exponential  equation  gives  in 
many  cases  values  of  S  smaller  than  the  observed  values, 
in  other  words  the  exponential  hypothesis  seems  to  err 
on  the  side  of  safety.1 

1  A  possible  explanation  of  the  increased  endurance  of  materials  under 
repetitions  of  low  stress  and  consequent  high  values  of  N  is  as  follows:  For 
high  stresses  the  damage  done  per  cycle  of  stress  covers  a  considerable  area 
of  cross-section  in  the  member  and  the  progress  of  structural  injury  pro- 
ceeds regularly.  For  low  stresses  little  isolated  areas  are  damaged,  and  the 
rapidity  with  which  damage  proceeds  is  a  "probability"  function  of  the 
grouping  of  the  damaged  areas.  It  has  been  suggested  in  a  paper  presented 
by  Moore  and  Seely  before  the  American  Society  for  Testing  Materials 
that  for  iron  and  steel  members  of  structures  and  machines  whose  failure 
would  not  endanger  life  the  values  of  the  stress  found  by  the  application 
of  the  exponential  hypothesis,  equation  (4)  or  equation  (5),  page  51,  may 
be  multiplied  by  a  "probability  factor"  greater  than  unity. 


+e 


-  20,000,000\ 


is  here  suggested  as  a  suitable  probability  factor,  in  which  the  constant  e  is 
2.718  the  base  of  the  hyperbolic  system  of  logarithms.     Values  of  this 
probability  factor  are  given  in  the  table  at  the  bottom  of  p.  50. 
4 


50  MATERIALS  OF  ENGINEERING 

In  view  of  the  fact  that  there  have  been  very  few  re- 
peated stress  tests  made  in  which  the  number  of  repetitions 
of  stress  exceeded  5,000,000,  that  up  to  1,000,000  repetitions 
of  stress  the  exponential  equation  seems  to  follow  test 
results  fairly  closely,  and  that  for  greater  numbers  of  repe- 
titions the  exponential  equation  seems  to  yield  results  on 
the  safe  side,  it  is  recommended  as  the  best  equation  we 
possess  at  present  for  computing  the  probable  strength  of 
materials  under  repeated  stress. 

Effect  of  Range  of  Stress. — Wohler  in  his  investigations 
of  the  effect  of  repeated  stress  discovered  that  the  number 
of  repetitions  of  stress  necessary  to  cause  rupture  depended 
on  the  range  of  unit  stress  during  each  cycle.  If  the  range 
of  unit  stress  was  from  zero  to  40,000  Ib.  per  square  inch 
fewer  repetitions  were  necessary  to  cause  rupture  than  if  the 
range  was  from  20,000  Ib.  per  square  inch  to  40,000  Ib. 
per  square  inch  (note  that  for  each  of  the  above  cases  the 
maximum  stress  is  the  same).  Various  formulas  have  been 
proposed  for  the  effect  of  range  of  stress  on  the  resistance 
to  repeated  stress.  From  a  study  of  the  work  of  Wohler 
and  the  supplementary  work  of  Bauschinger  the  effect  of 
range  of  stress  seems  to  be  fairly  well  expressed  by  the 
following  modification  of  equations  (2)  and  (3) :  For  the 

JD 

constant  K  substitute  the  expression  T~~Q  m  which  B 

is  a  constant  for  any  given  material  determined  from  re- 
peated stress  tests  and  Q  is" the  ratio  of  the  minimum  unit 
stress  during  a  cycle  to  the  maximum.  For  a  range  of 

N  Number  of  repetitions  of  stress  Probability  factor 


1,000,000 

1.000 

10,000,000 

1.135 

20,000,000 

1.368 

50,000,000 

1.670 

100,000,000 

1.818 

500,000,000 

1.960 

Infinity 

2.000 

REPEATED  STRESS  51 

unit  stress  from  zero  to  a  maximum,  Q  is  zero,  for  com- 
pletely reversed  stress  Q  is  —  1.  In  the  case  of  a  bridge 
chord,  in  which  the  unit  stress  due  to  live  load  is  four 
times  the  unit  stress  due  to  dead  load  and  is  in  the  same 
direction,  Q  is  +  0.2. 

The    exponential    equations   for   repeated    stress   then 
become: 


or 

log  S  =  log  B  -  log  (1  --  Q)  +m  log  N'  (4) 

Constants  for  the  Exponential  Equations  for  Repeated 
Stress.  —  As  our  experimental  data  for  repeated  stress  tests 
are  very  few,  values  of  constants  for  formulas  should  be 
regarded  as  tentative,  and  the  stresses  allowed  in  practice 
should  be  very  low.  The  repeated  stress  test  most  often 
made  is  of  a  rotating  shaft  under  a  bending  load;  this  test 
causes  complete  reversal  of  bending  stress.  Fig.  1  10  shows, 
in  diagram,  testing  machines  for  reversed  bending  stress. 
The  results  of  a  large  number  of  such  tests  of  specimens 
with  well-finished  surface  and  without  sharp  corners  at 
points  of  high  stress  are  fairly  well  expressed  by  the  formula  : 

£#-0.125 

r-^Q 

or 
log  S  =  log  B  -  log  (1  --  Q)  -  0.125  log  N  (5) 

in  which  B  is  a  constant  which  must  be  determined  ex- 
perimentally for  various  metals.  Some  values  of  B  obtained 
from  experiments  are  given  in  Table  3.  Poor  surface  finish, 
internal  cracks  or  flakes,  and  deep  scratches  or  tool  marks 
may  be  expected  to  have  the  effect  of  numerically  increas- 
ing the  exponent  of  M  above  the  value  0.125.1 

1  See  footnote,  page  49,  for  suggested  modification  of  formulas  for  low 
stresses  in  structures  whose  failure  does  not  involve  danger  to  life. 


52 


MATERIALS  OF  ENGINEERING 


TABLE  3. — VALUES  OF  THE  CONSTANT  B  IN  THE  EXPONENTIAL  FORMULA 
FOR  REPEATED  STRESS 


S 


i-Q 


or  log  S  =  log  B  -  0.125  log  N  -  log  (1 


This  equation  gives  rather  conservative  values  for  S  for  members  with 
shop-polished  surface,  and  with  all  corners  at  points  of  high  stress 
generously  filleted.  The  values  are  based  on  the  results  of  tests  from 
various  laboratories. 


Material 

B 

Log  B 

Structural  steel  and  soft  machinery  steel  
Wrought  irofi  
Steel  0.45  per  cent,  carbon  
Cold-rolled  steel  shafting  

250,000 
250,000 
350,000 
400,000 

5.39794 
5.39794 
5.54407 
5.60206 

Tempered  spring  steel 

400,000 
to 

5.60206 

Hard-steel  wire                                                       .  . 

800,000 
600,000 

5.90309 
5  77815 

Gray  cast  iron  

100,000 

5.00000 

Cast  aluminum 

80,000 

4  90309 

Hard-drawn  copper  

140,000 

5.14613 

For  tests  involving  repetitions  of  direct  tension  or  direct 
compression  we  have  even  less  data  than  for  reversed  bend- 
ing tests.  Such  data  as  we  have  indicate  that  the  value  of 
m  in  equations  (4)  and  (5)  is  numerically  slightly  larger 
than  that  given  by  repeated  bending  tests.  However, 
from  the  scanty  data  available  it  would  seem  that  for  mem- 
bers of  structures  or  machines  subjected  to  repetition  of 
•direct  tension  or  direct  compression  equation  (5)  with  the 
constants  given  in  Table  3  furnishes  a  fairly  safe  guide. 

1  In  the  formula  given  S  is  the  unit  stress  (Ib.  per  sq.  in.),  N  is  the  number 
of  repetitions  of  stress  necessary  to  cause  failure,  and  Q  is  the  ratio  of  the 
minimum  stress  applied  to  the  maximum  stress  (for  a  stress  varying  from 
zero  to  a  maximum,  Q  =  0;  for  a  completely  reversed  stress,  Q  =  —  1.0). 

This  equation  gives  results  for  unit  stress  lower  than  that  given  in  some 
tests  in  which  N  had  a  value  greater  than  1,000,000.  The  exponential 
formula  seems  to  err  somewhat  on  the  side  of  safety  for  a  large  number  of 
repetitions  of  stress.  On  page  49  in  a  footnote  is  given  a  suggested  "prob- 
ability factor"  which  may  be  used  in  connection  with  values  of  S  obtained 
by  the  above  equations.  It  is  recommended  that  for  structural  parts  or 
machine  members  whose  failure  would  endanger  life  that  the  exponential 
equation  be  used  unmodified  by  any  probability  factor. 


REPEATED  STRESS  53 

In  using  the  exponential  equations  for  repeated  stress 
it  must  always  be  borne  in  mind  that  under  no  considera- 
tion may  the  safe  static  stress  be  exceeded.  The  safe  static 
stress  is  a  criterion  independent  of  the  strength  under  re- 
peated stress.  As  an  illustration  suppose  that  N  be  taken 
as  less  than  100  for  structural  steel,  and  that  Q  betaken  as 
0.1,  the  application  of  equation  (5)  would  give  a  value 
of  S  higher  than  the  ultimate  static  strength  of  the  material. 
This  would  mean  that  for  the  conditions  given  static 
strength  would  govern  rather  than  strength  under  repeated 
stress. 

Diagrams  for  the  Exponential  Equations  for  Repeated 
Stress. — For  solving  the  exponential  equation  (5)  above, 
the  diagram  shown  in  Fig.  22  may  be  used.  The  manner 
of  using  the  diagram  is  as  follows : 

Enter  the  diagram  at  the  lower  edge  with  the  desired 
value  of  N  as  abscissa;  pass  verticaly  to  the  diagonal  line 
for  the  value  of  B  for  the  given  material  (if  the  exact  value 
of  B  for  the  material  is  not  plotted  the  location  of  its  line 
can  be  judged  by  interpolation  with  a  sufficient  degree  of 
accuracy);  then  pass  horizontally  to  the  diagnoal  line  for 
the  value  of  Q  corresponding  to  the  given  range  of  stress; 
then  vertically  to  the  upper  edge  of  the  diagram  where  the 
value  of  S  for  failure  under  repeated  stress  may  be  read 
from  the  scale. 

Wrought  Iron  versus  Steel. — In  general,  the  resistance 
of  a  material  to  repeated  stress  is  high  or  low  as  its  propor- 
tional limit  is  high  or  low,  though  there  seem  to  be  excep- 
tions to  this  rule,  due  largely  to  the  greater  sensitiveness  of 
special  alloys  to  slight  defects  in  heat-treating.  Wrought 
iron  has  a  lower  proportional  limit  than  does  structural 
steel,  and  the  resistance  to  repeated  stress  is,  in  general, 
less  for  wrought  iron  than  for  steel.  However,  the  unifor- 
mity and  reliability  of  wrought  iron  makes  it  superior  to 
some  or  the  cheaper  grades  of  steel — for  example  steel 
which  is  marked  as  "tank  steel/'  for  which  there  are 
no  standard  specifications,  and  which  is  very  variable  in 
quality. 


54 


MATERIALS  OF  ENGINEERING 


Effect  of  Rapidity  of  Repetition  of  Stress. — A  certain 
amount  of  time  is  required  for  any  member  of  a  machine  or 
structure  to  assume  the  deformation  corresponding  to  any 
given  load,  and  if  repetitions  of  load  follow  each  other  at 
intervals  shorter  than  this  time,  the  deformation  in  the 


Stress    (5)   Ib.  per   sq.  i 


in. 


§  §  §§'§: 


10 


Number  of    Repetitions    (N) 
To  cause  Failure 


FIG.  22.  —  Diagran   for  solving   the   exponential   equation   for  repeated    stress. 

BAT-o.m 

=  r^- 

member,  the  stress  set  up,  and  the  number  of  repetitions 
it  will  withstand  may  be  appreciably  affected.  A  few 
recent  British  tests  of  material  under  repeated  stress  seem 
to  indicate  that  for  small  members  there  is  no  appreciable 


REPEATED  STRESS  55 

effect  produced  by  varying  the  rapidity  of  repetition  of 
stress  below  about  2,000  repetitions  per  minute. 

Effect  of  Rest  on  Resistance  to  Repeated  Stress. — If 
metal  is  stressed  beyond  the  yield  point  so  that  plastic  action 
is  set  up,  its  strength  and  its  elastic  action  are  improved 
under  subsequent  stress,  if  the  material  is  allowed  to  rest 
(see  page  36).  Recent  experiments  by  British  investiga- 
tors and  by  Mr.  W.  J.  Putnam  of  the  University  of  Illinois 
seem  to  indicate  that,  for  steel  and  iron  at  least,  any  effect 
of  rest  on  the  resistance  to  repeated  stress  is  doubtful  for 
unit  stresses  below  the  yield  point  of  the  material. 

Effect  of  Sudden  Change  of  Outline  of  Member. — Every 
sharp  corner  in  a  piece  subjected  to  repeated  stress  facili- 
tates the  formation  of  micro-flaws  in  the  piece.  Results 
of  repeated  stress  tests  made  by  Stanton  and  Bairstow  at 
the  British  National  Physical  Laboratory  on  test  pieces 


Rounded  Fillet  Standard  Screw  Thread  Sharp  Corner 

'00%  70%  50% 

(Slight  rounding  of  Comers) 

FIG.  23. — Effect  of  sharp  corners  on  strength  under  repeated  stress. 

of  varying  shape  gave  the  following  relative  values  for 
strength  under  repeated  stress  for  the  shapes  tested : 

Rounded  fillet 100 

Standard  screw  thread 70 

Sharp  corner 50 

Fig.  23  shows  the  shapes  tested:  the  importance  of  avoid- 
ing sharp  corners  where  local  stresses  of  great  intensity  are 
set  up  is  very  great  for  members  subjected  to  repeated 
stress  on  account  of  the  danger  of  minute  cracks  forming 
and  spreading. 

Effect  of  Surface  Finish. — Rough  surface  finish  of  a  part 
subjected  to  repeated  stress  affords  opportunity  for  minute 
cracks  to  start  at  the  bottom  of 'scratches,  tool  marks,  or 
irregularities  due  to  scale.  Rough  surface  finish  may  re- 


56  MATERIALS  OF  ENGINEERING 

duce  the  stress  causing  failure  under  a  large  number  of 
repetitions  by  as  much  as  30  per  cent. 

Effect  of  Internal  Flaws  in  Structure.— Internal  flaws 
also  seem  to  reduce  the  resisting  power  of  metals  for  re- 
peated stress.  No  definite  quantitative  statement  can  be 
made  for  this  reduction,  but  metal  showing  flakes  and  other 
irregularities  in  fracture  should  be  looked  on  with  sus- 
picion if  intended  for  use  in  members  subjected  to  millions 
of  repetitions  of  high  stress. 

Service  Expected  from  Various  Machine  and  Structural 
Parts. — We  do  not  know  certainly  whether  any  material 
can  resist  an  infinite  number  of  repetitions  of  any  stress 
however  small.  The  safest  view  for  an  engineer  to  take 
seems  to  be  that  under  repeated  stress  materials  of  con- 
struction have  a  limited  "life."  The  exponential  equation 
for  resistance  to  repeated  stress  gives  results  in  accordance 
with  this  view.  If  this  view  is  held  the  number  of  repe- 
titions which  a  member  is  required  to  withstand  in  normal 
service  becomes  of  importance.  The  following  list  gives 
the  number  of  repetitions  for  various  structural  and  machine 
members.  The  list  is  intended  to  be  suggestive  rather  than 
to  serve  as  an  exact  guide. 

The  chord  members  of  a  railway  bridge  carrying  100  trains  per  day 
for  a  period  of  50  years  would  sustain  about  1,826,000  repetitions  of 
stress.  The  stress  would  vary  from  the  dead-load  stress  to  a  live-load 
averaging  somewhat  below  that  caused  by  the  passage  of  the  heaviest 
locomotives. 

The  floor  beams  in  an  elevated  railway  structure  during  a  period 
of  service  of  forty  years  are  subjected  to  about  40,000,000  repetitions 
of  stress;  the  variation  is  mainly  from  zero  to  a  maximum. 

A  railroad  rail  over  which  250,000,000  tons  of  traffic  passes  would 
sustain  something  like  500,000  repetitions  of  locomotive-wheel  loads, 
the  stress  being  slightly  more  severe  than  a  repetition  from  zero  to  a 
maximum.  The  rail  would  have  to  stand  in  addition  to  the  locomotive- 
wheel  loads  something  like  15,000,000  repetitions  of  stress  caused 
by  car-wheel  loads.  The  stresses  set  up  by  car-wheel  loads  would  be 
about  half  as  great  as  the  stresses  set  up  by  the  locomotive-wheel 
load. 

A  mine-hoist  rope  bent  over  three  sheave  wheels  and  operating  a 
hoist  100  times  a  day,  in  a  term  of  service  of  5  years  would  sustain 


REPEATED  STRESS  57 

550,000  repetitions  of  stress.  If  the  sheave  wheels  are  so  placed  that 
they  reverse  the  direction  of  the  bending  of  the  rope  they  would  nearly 
cause  a  complete  reversal  of  stress;  if  bending  takes  place  in  one  direc- 
tion only  the  range  of  stress  is  from  nearly  zero  to  a  maximum. 

The  piston  rod  and  the  connecting  rod  of  a  steam  engine  running 
at  300  r.p.m.  for  10  hr.  per  day,  300  days  per  year  for  10  years  sustains 
540,000,000  repetitions  of  stress,  and  the  range  of  stress  involves 
almost  complete  reversal. 

A  band  saw  in  hard  service  for  2  months  sustains  about  10,000,000 
repetitions  of  stress  varying  from  nearly  zero  to  a  maximum. 

A  line  shaft  running  at  250  r.p.m.  for  10  hr.  a  day,  300  days  per 
year,  sustains  during  a  service  of  20  years  900,000,000  repetitions  of 
bending  stress  due  to  force  transmitted  by  belts,  gears,  and  driving 
chains.  The  stress  is  completely  reversed.  It  should  be  noted  that 
for  the  line  shaft  the  torsional  stress  is  not  repeated  nearly  so  often  as 
is  the  bending  stress. 

The  shaft  of  a  horizontal  steam  turbine  running  at  3,000  r.p.m. 
for  24  hr.  per  day,  365  days  in  a  year  during  10  years'  service  sustains 
15,768,000,000  reversals  of  bending  stress  caused  by  the  weight  of 
rotating  parts  and  the  tangential  force  of  the  inrushing  steam. 

The  crankshaft  of  an  automobile  engine  is  subjected  to  about  120,- 
000,000  cycles  of  stress  by  the  time  the  car  has  run  50,000  miles,  and  the 
stress  is  nearly  complete  reversal. 

The  crankshaft  of  an  airplane  engine  in  150  hours  of  actual  flying 
is  subjected  to  about  18,000,000  cycles  of  stress,  the  stress  being  almost 
completely  reversed.  An  airplane  engine  runs  at  nearly  full  power  all 
the  time,  and  this  makes  the  reversals  of  stress  especially  destructive. 

It  should  be  noted  that  other  factors  than  breakdown  of 
the  material  by  repeated  stress  tend  to  cause  parts  of  ma- 
chines and  structures  to  have  a  limited  "life."  Wear 
caused  by  friction  of  rubbing  parts  is  a  very  common  cause 
limiting  the  "life"  of  a  machine  part.  An  illustration  of 
this  action  is  furnished  by  railroad  rails  in  which  the  head 
usually  wears  out. under  the  action  of  car  and  locomotive 
wheels  before  the  rail  is  in  danger  of  failure  by  repeated 
stress. 

The  foregoing  discussion  concerns  itself  with  stresses 
which  by  their  repeated  application  cause  failure.  Safe 
working  stresses  for  members  subjected  to  repeated  stress 
are  discussed  in  Chapter  IV. 


58  MATERIALS  OF  ENGINEERING 


Selected  References  for  Further  Study 

UNWIN:  "The  Testing  of  the  Materials  of  Construction,"  London,  1910, 
Chap.  XVI. 

JOHNSON:  "The  Materials  of  Construction,"  (rewritten  by.Withey  and 
Aston,  edited  by  Turneaure),  New  York,  1918,  Chap.  XXVIII. 
(Written  by  Kommers.) . 

BASQUIN:  The  Exponential  Law  of  Endurance  Tests,  Proceedings  of  the 
American  Society  for  Testing  Materials,  Vol.  X,  p.  625  (1910). 

ST ANTON  and  BAIRSTOW:  On  the  Resistance  of  Iron  and  Steel  to  Reversals 
of  Stress,  Proceedings  of  the  Institute  of  Civil  Engineers  (British)  Vol, 
CLXVI,  p.  78  (1906).  See  also  Bairstow,  Phil  Trans.  Royal  Soc.  A., 
Vol.  CCX,  p.  35  (1910). 

UPTON  and  LEWIS:  The  Fatigue  Failure  of  Metals,  American  Machinist, 
Oct.  17  and  24,  1912. 

MOORE  and  SEELY:  The  Failure  of  Materials  under  Repeated  Stress,  Pro- 
ceedings of  the  American  Society  for  Testing  Materials,  Vol.  XV, 
Part  II,  p.  438  (1915).  Constants  and  Formulas  for  Repeated  Stress 
Calculations,  Proceedings  of  the  American  Society  for  Testing  Materials 
Vol.  XVI,  Part  II,  p.  471  (1916).  The  1916  paper  corrects  a  numerical 
error  in  the  1915  paper. 

KOMMERS:  "Broader  Use  of  Johnson's  Formula  for  Repeated  Stress," 
Eng'g  News-Record,  Vol.  LXXXIII,  No.  20,  p.  942. 

THE  NATIONAL  RESEARCH  COUNCIL  COMMITTEE  ON  FATIGUE  PHENOMENA 
IN  METALS:  Report  of  Progress  in  the  September,  1919,  issue  of 
"Mechanical  Engineering"  (the  publication  of  the  Am.  Soc.  of  Mech. 
Engrs.). 


CHAPTER  IV 

WORKING  STRESS ;  FACTOR  OF  SAFETY ; 
SELECTION  OF  MATERIALS 

Working  Stress. — In  the  study  of  mechanics  of  materials 
stress  is  considered  as  an  internal  force  in  a  body  which 
resists  external  forces  acting  on  the  body.  In  dealing 
with  members  of  structures  and  machines  it  is  needful  to 
hold  this  in  view,  and  to  consider  also  the  destructive  effect 
of  stress,  and  the  accompanying  strain,  on  the  materials 
used.  Two  questions  must  be  answered:  (1)  What  is  the 
maximum  unit  stress  set  up  in  the  body?  This  is  a  ques- 
tion to  be  answered  by  mathematical  analysis.  (2)  Does 
this  unit  stress  exceed  the  safe  stress  for  the  material 
used?  This  question  demands  for  its  answer  a  knowledge 
of  the  physical  properties  of  materials. 

In  considering  the  safety  under  load  of  a  structure  or 
a  machine,  several  viewpoints  are  possible.  The  designer 
wishes  to  know  how  large  the  members  must  be  to  carry 
the  given  load;  the  purchaser  wishes  to  know  how  great 
a  load  can  be  safely  carried  by  the  machine  or  the  struc- 
ture; the  inspector  wishes  to  know  how  great  stresses  are 
set  up  by  the  loads  actually  imposed  on  the  structure  or 
the  machine.  All  these  viewpoints  involve  a  considera- 
tion of  the  stress  set  up  by  the  working  load,  that  is,  of  the 
working  stress.  A  safe  working  stress  for  the  material  in 
a  structure  or  a  machine  may  be  defined  as  a  unit  stress 
(measured  in  pounds  per  square  inch)  which  under  the 
conditions  of  operation  or  use  will  not  cause  structural 
damage. 

The  maximum  safe  working  stress  for  any  material  is 
always  less,  much  less,  than  the  ultimate  strength  of  the 
material  (as  determined  by  tests  of  sample  test  specimens) . 
Four  general  reasons  may  be  given  for  this  fact : 

59 


60  MATERIALS  OF  ENGINEERING 

1.  A  structure  or  machine  would  not  give  satisfactory 
service   if   it  were  on  the  point  of  failure.     Nearly  all 
materials  show  marked  distortion  before  failure,  and  the 
stiffness  is  greatly  reduced. 

2.  Complete  information  as  to  the  properties  of  the 
material  in  any  actual  machine  or  structure  is  never  avail- 
able.    Sometimes    the    process    of    manufacture    of   the 
material  is  not  known  (for  example,  it  may  not  be  known 
whether  steel  is  Bessemer  steel  or  open-hearth  steel;  whether 
concrete  is  hand-mixed  or  machine-mixed) .     The  thorough- 
ness of  testing  and  of  inspection  of  material  varies  widely 
for  different  classes  of  work.     If  the  material  is  a  part 
of  a  large  shipment  the  inspection  and  testing  may  have 
been  very  thorough ;  if  the  shipment  was  a  small  one  there 
may  have  been  no  inspection  at  all.     The  process  of  fabrica- 
tion of  the  material  may  have  weakened  it,  as  is  the  case 
with  steel  plates  or  shapes  in  which  holes  have  been  punched 
for  rivets,  or  which  have  been  bent  or  hammered  into 
position  without  being  heated.     The  material  may  suffer 
deterioration  as  time  elapses;  for  example,  steel  exposed 
to  moist  air  or  to  smoke  corrodes  unless  kept  thoroughly 
covered  with  a  coat  of  paint;  concrete  may  be  weakened 
by  electrolysis  from  stray  currents  from  street-car  power 
circuits;  wood  decays. 

3.  The  magnitude  of  the  loads  actually  applied  to  a 
machine  or  a  structure,  and  the  magnitude  of  the  stresses 
set  up  are  never  known  exactly.     The  loading  actually 
placed  on  the  floor  of  a  building  or  on  the  roadway  of  a 
bridge,  or  the  actual  working  load  on  a  machine  will  not, 
in  general,  be  exactly  that  assumed  in  the  design.     The 
fitting  of  members  to  each  other  is  never  perfectly  done, 
and  the  distribution  of  stress  among  members  is  not  in- 
frequently markedly  different  from  that  assumed  in  the 
design.     For  example,  for  a  group  of  eyebars  making  up  a 
bridge  chord,  the  load  is  generally  assumed  to  be  equally 
distributed  among  them,   actual  measurements  of  strain 
sometimes  show  considerable  differences  between  the  loads 
carried  by  different  eyebars  of  the  group.     The  uncertainty 


WORKING  STRESS  61 

of  distribution  of  stress  between  members  is  especially 
marked  in  " statically  indeterminate''  structures,  such  as 
fixed-ended  beams,  trusses  with  " redundant"  members, 
and  the  like. 

The  science  of  the  mechanics  of  materials  is  not  yet  com- 
•pletely  developed.  The  laws  of  stress  in  many  common 
structural  members  are  not  yet  fully  known;  for  example, 
the  equations  commonly  used  in  designing  columns  are 
still  largely  of  an  empirical  nature,  different  methods  of 
computing  stresses  in  curved  beams  or  in  thick  cylinders 
yield  varying  results. 

Different  kinds  of  loading  produce  varying  degrees  of 
structural  distress.  A  load  repeated  millions  of  times  is 
far  more  disastrous  than  is  a  steady  load,  and  to  some 
materials  (notably  wood)  a  long-continued  load  is  more 
destructive  than  is  one  of  short  duration.  The  damage 
caused  by  shock  incident  to  the  operation  of  machines 
very  often  can  not  be  computed  with  any  degree  of 
exactness. 

4.  Any  machine  or  structure  is  liable  to  be  subjected  to 
an  accidental  or  a  temporary  overload,  and  some  margin 
of  safety  should  be  left  to  provide  for  this  contingency. 
A  good  illustration  of  such  overloads  is  furnished  by  hoist- 
ing chains  and  ropes,  which  frequently  are  subjected  to 
excessive  load  on  account  of  the  slipping  of  hooks  and 
hitches. 

Consequence  of  Failure  of  Material. — In  determining 
the  maximum  safe  working  stress  for  any  machine  member 
or  structural  part,  the  consequences  of  failure  should  be 
taken  into  consideration.  If  failure  of  the  piece  endangers 
human  life,  the  allowable  working  stress  is  to  be  taken 
less  than  for  a  similar  piece  whose  failure  involves  merely 
material  damage.  If  the  failure  of  a  particular  piece  will 
not  cause  complete  collapse  of  the  structure,  and  if  it 
can  be  readily  replaced,  a  relatively  high  working  stress 
may  be  allowed. 

Certain  types  of  machine  and  structural  members  do 
not  normally  carry  any  great  amount  of  stress  in  service, 


62  MATERIALS  OF  ENGINEERING 

but  are  provided  for  the  purpose  of  insuring  the  machine 
or  structure  against  sudden,  destructive  collapse  if  the 
main  members  fail.  Spiral-steel  reinforcement  in  concrete 
columns  furnishes  an  excellent  illustration  of  this  "  in- 
surance" under  a  load.  If  a  non-reinforced  concrete 
column  is  loaded  to  the  ultimate,  a  shattering  collapse 
takes  place,  while  if  the  column  is  reinforced  by  a  spiral 
of  steel  the  same  load  causes  a  large  distortion,  but  the 
failure  is  a  gradual  one  instead  of  sudden,  complete  collapse. 

Factor  of  Safety. — A  method  which  has  been  widely 
employed  for  determining  the  maximum  allowable  working 
stress  for  a  material  consists  in  dividing  the  ultimate 
strength  of  the  material  (as  determined  by  laboratory 
tests  of  specimens)  by  a  "  factor  of  safety. "  Values  of  this 
factor  in  common  use  vary  all  the  way  from  2.5  for  steel 
under  steady  load  to  15  or  20  for  timber  under  repeated 
load  or  shock.  The  use  of  this  term  " factor  of  safety" 
not  infrequently  gives  the  designer  or  the  purchaser  a  false 
sense  of  security.  If  a  structure  is  designed  with  a  factor 
of  safety  of  5  it  is  by  no  means  certain  that  it  will  stand  up 
under  five  times  the  working  load.  The  factor  of  safety 
is  really  more  of  a  factor  of  uncertainty  than  a  factor  of 
safety. 

Standard  Allowable  Working  Stresses. — In  the  drafting 
room  of  a  machine-building  plant  or  of  a  large  structural 
firm  allowable  working  stresses  for  the  common  materials 
become  fixed  by  experience.  For  example,  it  has  become 
very  general  practice  to  allow  under  steady  load  a  working 
tensile  stress  of  16,000  Ib.  per  square  inch  for  structural 
steel  rolled  into  rods,  plates,  or  beams.  In  building  con- 
struction, standardization  of  working  stresses  has  pro- 
ceeded so  far  that  in  the  building  laws  of  most  large  cities 
the  maximum  allowable  working  stresses  for  the  common 
structural  materials  are  definitely  fixed.  Table  4  gives  a 
summarized  statement  of  some  such  allowable  stresses. 
Technical  committees  composed  of  members  of  the  leading 
engineering  societies  have  also  formulated  codes  giving 
allowable  stresses  for  some  structural  materials. 


WORKING  STRESS 


63 


TABLE  4. — WORKING  STRESSES  FOR  STRUCTURAL  MATERIALS 

The  values  given  in  this  table  are  based  on  a  comparison  of  the  values 
given  in  the  building  ordinances  of  New  York,  Chicago,  Philadelphia  and 
Boston. 


Material 

Kind  of  stress 

Allowable  stress, 
Ib.  per  sq.  in. 

Ordinary  rubble  masonry: 
Portland-cement  mortar  

Compression  (bearing)  .  . 

100 

Lime  mortar     ....          .... 

60 

Coursed  rubble  masonry; 

200 

Lime  mortar       .        

Compression  (bearing) 

120 

Squared  masonry,  Portland-cement  mortar: 

600 

Limestone  or  sandstone  

Compression  (bearing)  . 

400 

Portland-cement  concrete: 
1  cement;  6  aggregate,  machine-mixed.  .  . 
1  cement;  6  aggregate,  hand-mixed  

Compression  (bearing)  .... 
Compression  (bearing)  .... 

400 
350 

1  cement;  9  aggregate,  machine-mixed 
1  cement;  9  aggregate,  hand-mixed  

Compression  (bearing)  .... 
Compression  (bearing)  .... 

300 
250 

Brick  masonry: 
Paving  brick,  Portland-cement  mortar.  .  . 
Pressed  brick,  Portland-cement  mortar.  .  . 
Common  brick,  lime  mortar  
Common  brick,  Portland-cement  mortar.. 

Compression    bearing)  .... 
Compression    bearing)  .... 
Compression    bearing)  .... 
Compression    bearing)  .... 

350 
250 
100 
175 

Wood: 

1  100 

Yellow  pine     

Compression  along  grain 

1  000 

250 

Yellow  pine  

Shear  along  grain    

100 

Douglas  fir  
Douglas  fir 

Tension  .fibers  in  beams.  .  . 

1,300 
1  100 

Douglas  fir  

Compression  across  grain 

250 

Douglas  fir 

115 

Norway  pine,  white  pine,  spruce  
Norway  pine,  white  pine,  spruce.  .  .  '.  
Norway  pine,  white  pine,  spruce  

Tension  fibers  in  beams  .  .  . 
Compression  along  grain  .  . 
Compression  across    grain  . 

900 
750 
250 
60 

Hemlock 

750 

Hemlock  

Compression  along  grain    . 

500 

Hemlock 

150 

Hemlock  

Shear,  along  grain  

50 

Structural  steel  

Tension  

16,000 

Structural  steel                         .  .    . 

Compression 

15,000 

Structural  steel 

Shear 

10  000 

Rivet  steel 

Shear 

9  000 

Steel  of  rivets  and'  pins 

Bearing 

25  000 

Steel  of  rivets  and  pins  

Extreme  stress  in  bending  . 

25,000 

Steel  castings 

16  000 

Steel  castings.          

Compression  .  . 

15,000 

Wrought  iron  

Tension  

12,000 

Wrought  iron 

Compression 

11  000 

Wrought  iron  

Shear  

6,000 

Cast  iron  

Tension  in  beams  

3,000 

Cast  iron  

Compression 

13,000 

Cast  iron 

Shear 

3  000 

64  MATERIALS  OF  ENGINEERING 

In  machine  building,  standardization  of  working  stresses 
is  more  difficult  than  in  structural  work;  the  variety  of 
materials  used  is  greater,  and  the  conditions  of  service 
less  certain.  In  a  large  manufacturing  establishment 
certain  standard  allowable  stresses  are  soon  developed  for 
its  particular  line  of  work.  The  tables  of  allowable  stresses 
and  of  working  formulas  given  in  the  various  handbooks 
for  machine  and  structural  designers  give  valuable  informa- 
tion as  to  allowable  stresses,  but  such  tables  must  not  be 
used  blindly  without  any  consideration  of  their  limitations. 
The  study  of  the  proportions  of  and  probable  stresses  in 
successful  machines  is  often  the  best  guide  for  the  machine 
designer  or  the  purchaser  in  cases  where  practice  has  not 
yet  standardized  allowable  working  stresses. 

Working  Stresses  for  Material  Subjected  to  Repeated 
Loading. — For  structural  or  machine  parts  subjected 
to  repeated  loading  the  working  stresses  allowed  must 
evidently  be  less  than  the  unit  stresses  given  by  equation 
(1)  or  equation  (5)  Chapter  III,  which  are  stresses  for 
failure. 

In  choosing  working  stresses  for  members  subjected  to 
repeated  stress  it  should  be  remembered  that  a  small  re- 
duction in  stress  very  greatly  increases  the  number  of 
repetitions  of  stress  which  the  material  can  withstand.  For 
metals  it  seems  from  test  results  that  a  decrease  of  9  per 
cent,  in  stress  nearly  doubles  the  "life"  of  the  material. 
Since  the  endurance  of  a  material  is  so  sensitive  to  changes 
in  the  magnitude  of  stress,  it  seems  more  logical  for  re- 
peated-stress problems  to  apply  the  "factor  of  safety" 
used  to  the  number  of  repetitions  rather  than  to  the  stress, 
computing  the  probable  stress  at  failure  for  a  number  of 
repetitions  many  times  greater  than  the  number  which  the 
member  is  expected  to  withstand  in  service.  If  the  factor 
of  safety  is  applied  to  N,  then  it  should  be  very  much  larger 
than  the  factors  commonly  applied  to  stresses  in  static- 
stress  problems.  The  test  data  for  repeated  stress  is  very 
much  less  extensive  than  the  test  data  for  static  strength, 
and  the  test  results  show  wide  variation  in  N  for  small 


WORKING  STRESS  65 

change  in  S;  and  while  the  equations  given  yield  results  a 
little  lower  than  the  average  results  of  tests,  yet  for  some 
few  tests  failure  occurred  at  lower  values  of  N  than  the 
equations  would  indicate.  To  guard  against  this  variation, 
and  against  the  large  variation  in  N  which  would  be  caused 
by  slight  variations  in  the  stress  actually  applied  to  the 
material,  a  factor  of  safety  of  100  would  not  seem  too  large, 
if  applied  to  the  number  of  repetitions  of  stress. 

In  problems  involving  static  strength  it  is  customary  to 
apply  the  factor  of  safety  to  the  stress  which  will  cause 
failure.  If  this  practice  is  followed  for  repeated-stress 
problems  (remembering  always  that  under  any  conditions 
the  stress  allowed  must  not  be  greater  than  the  safe  static 
working  stress  for  the  material)  a  factor  of  safety  of  1.8 
would  correspond  to  an  increase  of  endurance  of  something 
over  100  times. 

Materials  for  Various  Classes  of  Machines  or  Struc- 
tures.— Some  illustrations  of  the  selection  of  materials  for 
machine  and  structural  parts  will  be  given  in  this  section. 
The  selections  noted  are  to  be  regarded  as  typical  rather 
than  as  furnishing  anything  like  a  complete  list. 

Bridges.- — The  materials  usually  available  for  bridge 
work  are  timber,  concrete,  steel,  and  stone.  For  short- 
span  bridges  timber  usually  is  the  cheapest,  the  lightest  and 
the  least  durable  material.  For  temporary  structures, 
military  bridges  and  bridges  on  roads  to  logging  camps  or 
contractors'  camps,  timber  is  the  material  usually  used. 
For  permanent  bridges  timber  is  now  rarely  used.  If  the 
bridge  is  long-span  bridge,  steel  is  in  nearly  all  cases  the 
best  material  to  use,  since  in  a  long-span  bridge  the  dead 
weight  of  the  material  furnishes  a  large  part  of  the  total 
load,  and  for  a  given  strength  a  steel  structure  is  lighter 
than  one  of  concrete  or  stone.  For  very  long  spans  nickel 
steel  or  other  special  steel  of  high  strength  is  frequently 
used.  For  short-span  bridges  both  steel  and  concrete  are 
used.  The  relative  cost  and  the  degree  of  permanence  of 
the  work  varies  with  the  location,  the  surroundings,  and 
the  character  of  the  traffic.  A  well-made  reinforced- 


66  .     MATERIALS  OF  ENGINEERING 

concrete  bridge  suffers  less  deterioration  and  costs  less  for 
upkeep  than  does  a  steel  bridge,  and  is  subject  to  less 
vibration  under  heavy  loads.  For  the  floor  of  the  bridge, 
timber  and  reinforced  concrete  are  used;  the  use  of  rein- 
forced concrete  is  becoming  more  and  more  common.  A 
combination  of  bridge  materials  frequently  used  is  steel  for 
the  main  trusses  or  beams,  and  for  floor  beams;  and  rein- 
forced concrete  for  the  floor  of  the  bridge.  Stone  masonry 
bridges  are  built  for  locations  involving  short  spans,  where 
appearance  and  permanence  are  factors  overbalancing 
first  cost. 

Buildings. — Timber  is  used  for  temporary  construction, 
and  for  buildings  for  which  low  cost  is  the  prime  considera- 
tion. For  the  framework  of  very  high  buildings  steel  is 
the  material  nearly  always  used.  For  the  beams  of  fac- 
tories, warehouses,  and  public  buildings  steel,  reinforced 
concrete,  and  timber  are  used.  For  the  columns  the  above 
materials  are  used,  also  for  short  columns,  terra-cotta,  brick, 
and  cast  iron.  For  walls  brick,  concrete  blocks  and  stone 
masonry  are  used  and  for  light  walls  which  do  not  have  to 
carry  much  stress  timber  with  plaster  on  laths,  terra-cotta 
and  gypsum  are  used.  As  in  the  case  of  bridges,  stone 
masonry  is  used  where  fine  appearance  and  permanence 
overbalance  considerations  of  first  cost. 

Shafting. — Strength,  stiffness,  and  compactness  are  very 
necessary  properties  in  shafting,  and  steel  is  the  material 
in  almost  universal  use.  Cold-rolled  steel  is  very  widely 
used  on  account  of  the  ease  and  cheapness  with  which  it 
can  be  rolled  true  to  shape  and  to  size.  For  shafting  to  be 
used  in  machine  tools  or  other  machines  in  which  accuracy 
of  motion  is  of  prime  importance,  cold-rolled  shafting  is  too 
liable  to  " spring"  out  of  shape  if  machined,  on  account  of 
the  heavy  internal  stresses  set  up  in  it  by  the  cold-rolling 
process.  For  such  machines  turned  steel  shafting  is  used. 
For  very  hard  service,  automobile  shafts,  screws  for  screw 
presses,  etc.,  special  alloy  steels  of  high  strength  are  used. 
It  should  be  remembered,  however,  that  if  the  working 
stresses  are  low,  a  shaft  of  ordinary  low-carbon  steel  is  as 


WORKING  STRESS  67 

stiff  as  one  of  high-carbon  steel  or  of  special  alloy  steel  (see 
page  38). 

Railway  Equipment. — Couplers,  yokes  (for  holding  the 
couplers)  and  bolsters  (for  carrying  the  bearings)  are  usu- 
ally made  from  steel  castings.  The  complicated  shape 
required  makes  it  expensive  to  use  pieces  built  up  of  rolled 
steel,  and  under  the  repeated  pounding  of  railroad  service 
every  joint  in  a  built-up  member  is  a  source  of  possible 
weakness.  The  recent  improvements  in  the  process  of 
casting  and  heat-treating  steel  places  this  material  in  a 
position  of  first  importance  for  railway  appliances.  Loco- 
motive frames  are  sometimes  steel  castings  and  sometimes 
forgings.  Brake  beams  for  locomotives  and  cars  are  usu- 
ally built  up  from  rolled  shapes,  and  rods.  For  railway 
service  toughness  is  an  important  quality  for  materials  on 
account  of  the  frequent  heavy  shocks  to  which  equipment 
is  subjected. 

Car  Wheels. — The  requirements  for  the  material  in  the 
different  parts  of  a  railway  car  wheel  are  diverse.  The 
tire  must  be  hard  to  resist  wear;  the  hub  and  plate  (or 
spokes)  should  not  be  brittle  on  account  of  the  shock 
which  must  be  withstood.  Two  solutions  for  these  di- 
verse requirements  are  common:  (1)  the  wheel  is  made  of 
cast  iron,  the  hub  and  plate  being  soft  gray  iron,  while  the 
rim  is  " chilled"  and  consequently  hard;  and  (2)  the  wheel 
is  made  of  rolled  steel  of  a  grade  which  combines  consider- 
able hardness  with  a  fair  degree  of  ductility.  Both  cast- 
iron  car  wheels  and  steel  car  wheels  have  their  advocates, 
and  both  are  in  extensive  use. 

Machine  Frames. — For  machine  frames  in  which  stiffness 
and  steadiness  against  vibration  are  the  prime  requisites 
and  in  which  weight  is  not  objectionable  (for  exam- 
ple, the  frame  of  a  steam  engine  or  of  a  machine  tool) 
cast  iron  is  the  material  usually  used  on  account  of  its 
low  cost.  For  machine  frames  carrying  heavy  stresses  (for 
example,  punch  and  shear  frames)  cast  iron  is  sometimes 
used,  frequently  the  special  cast  iron  known  as  " semi-steel" 
is  used,  and  sometimes  steel  castings  are  used.  The  use  of 


68  MATERIALS  OF  ENGINEERING 

steel  castings  for  heavily  stressed  frames  is  constantly 
increasing. 

Engine,  Pump,  and  Hydraulic-press  Cylinders. — For 
cylinders  carrying  low  pressures — engine  cylinders,  air- 
compressor  cylinders,  and  low-pressure  pump  cylinders  — 
cast  iron  is  the  material  commonly  used.  For  such  cylin- 
ders the  thickness  is  determined  not  by  considerations  of 
strength  but  by  considerations  of  foundry  practice — it  is 
not  possible  to  cast  a  very  thin  cylinder.  Cast  iron  makes 
a  much  better  bearing  metal  for  the  rubbing  of  the  piston 
than  does  steel.  For  high-pressure  cylinders,  for  example, 
cylinders  for  hydraulic  presses,  steel  castings  or  steel  forg- 
ings  are  used  because  of  their  ability  to  resist  the  high 
stresses  which  are  set  up.  For  such  cylinders  the  speed 
of  rubbing  of  piston  over  cylinder  is  usually  low  and  the 
effect  of  the  high  friction  of  steel  on  steel  is  not  so  im- 
portant. Where  there  is  great  danger  of  corrosion,  brass 
or  bronze  linings  are  frequently  used  with  steel  cylinders, 
and  the  small  cylinders  of  high-pressure  hydraulic  pumps 
are  frequently  made  of  bronze  castings.  Bronze  is  expen- 
sive, but  it  combines  strength,  good  bearing  qualities  and 
resistance  to  corrosion. 

Bearings.- — The  material  for  bearings  usually  is  chosen 
for  it  smooth-wearing  qualities  rather  than  for  its  strength, 
though  for  bearings  in  machines  subjected  to  heavy  static 
loads,  or  to  severe  impact,  bearings  must  be  made  of 
material  having  a  good  degree  of  compressive  strength. 
Steel  makes  a  poor  bearing  material  for  steel  shafts  or 
slides.  Steel  rubbing  on  steel  develops  a  rough,  torn  sur- 
face which  soon  becomes  hot.  Cast  iron  makes  a  much 
better  bearing  metal  for  steel  shafts  than  does  steel,  though 
its  brittleness  limits  its  use  to  bearings  not  subjected  to 
shock.  Bronze  is  an  excellent  bearing  metal  especially 
for  heavy  loads.  Numerous  "soft"  bearing  alloys  are  in 
use.  Prominent  among  these  is  Babbitt  metal,  an  alloy 
of  antimony,  tin,  and  copper.  Other  alloys  contain  vary- 
ing proportions  of  antimony,  tin,  and  lead.  These  soft 
bearing  metals  are  widely  used  for  bearings  which  run  under 


WORKING  STRESS  69 

light  steady  pressure.  These  soft  alloys  can  be  readily 
melted  and  cast  in  place  round  a  shaft,  and  in  case  the 
metal  becomes  accidentally  overheated  and  melted  out  it 
can  be  readily  replaced.  The  soft  bearing  metals  are  not 
suitable  for  use  under  heavy  pressures,  which  cause  them 
to  "flow." 

Selected  References  for  Further  Study 

Any  detailed  list  of  references  for  allowable  stresses  in  structures  and 
machines  would  cover  several  pages  of  this  book.  In  general,  such  refer- 
ences would  include:  Pocket  Books  and  Hand  Books  for  Civil,  Mechanical 
and  Architectural  Engineers;  Building  Ordinances  of  Cities;  Handbooks  of 
Steel  Manufacturing  Companies;  and  Reports  of  Committees  of  Engineering 
Societies. 


CHAPTER  V 
THE  MANUFACTURE  OF  PIG  IRON 

Occurrence  of  Iron  in  Nature. — Iron  is  found  in  nature 
in  the  form  of  vast  deposits  of  iron  ores,  most  of  which  are 
iron  oxide.  It  is  possible  to  produce  nearly  pure  iron  di- 
rectly from  the  ore  by  the  removal  of  oxygen  from  iron 
oxide1  (this  process  is  called  reduction  of  the  ore) ;  it  has 
been  found  more  economical,  however,  to  first  reduce  the 
ore  to  an  impure  iron  containing  a  high  percentage  of  carbon. 
This  reduction  takes  place  in  a  tall  stack  known  as  a  blast 
furnace.  The  impure  iron  produced  in  a  blast  furnace  is 
known  as  pig  iron;  and  this  pig  iron  may  be  further  refined 
by  means  of  an  open-hearth  furnace,  a  Bessemer  converter, 
a  puddling  furnace,  or  an  electric  furnace.  Descriptions 
of  these  refining  processes  are  given  in  succeeding  chapters. 

Ores  of  Iron. — The  principal  ore  of  iron  is  hematite 
(ferric  oxide  Fe2O3);  other  ores  are  magnetite  (Fe3O4)  and 
siderite  or  spathic  ore  (FeCO3).  The  value  of  an  iron  ore  is 
determined  largely  by  the  percentage  of  iron  it  contains. 
Ores  carrying  50  per  cent,  or  more  of  iron  are  known  as 
high-grade  ores.  Ores  carrying  less  than  50  per  cent,  of 
iron  are  called  low-grade  ores.  The  value  of  an  ore  is  also 
affected  by  the  nature  of  the  impurities  it  carries.  Com- 
mon impurities  present  in  iron  ore  are  sulphur  (as  sulphides), 
phosphorus  (as  phosphates)  silica  and  earthy  matter. 
Sulphur  and  phosphorus  are  especially  undesirable  elements 
in  iron  ore. 

Fig.  24  shows  the  location  of  the  principal  iron  ore  de- 
posits in  the  United  States.  About  85  per  cent,  of  the  ore 

1  In  earlier  times  direct  reduction  of  ore  to  nearly  pure  iron  was  the  com- 
mon method  used  in  producing  iron,  and  today  there  is  some  promise  of 
commercial  usefulness  for  a  process  producing  pure  iron  direct  from  iron  ore 
by  means  of  the  electric  furnace  (see  page  107).  The  direct  production  of 
iron  from  ore  is,  however,  mainly  of  historical  interest. 

70 


THE  MANUFACTURE  OF  PIG  IRON 


71 


mined  in  this  country  comes  from  the  Lake  Superior  region 
and  is  red  hematite.     The  ore  of  the  Alabama  region  is  of 


lower  grade  than  that  from  the  Lake  Superior  region,  but 
the  Alabama  region  is  the  next  in  importance  to  the  Lake 
Superior  region  in  amount  of  production.  The  purest  ores 


72  MATERIALS  OF  ENGINEERING 

in  the  world  are  found  in  Sweden,  and  the  pig  iron  and 
pure  iron  reduced  from  those  ores  is  of  the  highest  quality. 
The  annual  output  of  iron  ore  for  the  United  States  in 
normal  times  is  about  thirty  million  tons. 

Mining  and  Preparation  of  Iron  Ore. — Most  American 
ores  of  iron  lie  near  the  surfac'e  of  the  ground,  and  are 
mined  by  open-surface  mining.  The  red  hematite  ores, 
which  are  the  most  important,  are  soft  earthy  ores  and  are 
mined  with  steam  shovels.  Some  harder  ores  require 
drilling  and  blasting.  The  red  hematite  ores  require  no 
special  preparation  before  they  are  fed  to  the  blast  furnace 
for  reduction.  Other  ores  carrying  more  impurities  require 
preliminary  treatment.  Ores  in  the  form  of  iron  carbonate 
are  usually  transformed  to  iron  oxide  by  heating,  and  ores 
containing  a  high  percentage  of  water  are  heated  to  drive 
off  this  water.  Ores  containing  a  high  sulphur  content 
are  roasted  at  a  temperature  high  enough  to  burn  off  the 
sulphur. 

Reduction  of  Ore  to  Pig  Iron. — Iron  ore  is,  in  general, 
iron  oxide  and  to  be  changed  to  metallic  iron  it  must 
be  deoxidized  or  reduced.  The  reducing  agent  is  carbon, 
which  is  also  used  as  fuel,  and  in  the  reducing  process  the 
iron  is  produced  in  a  molten  condition,  in  which  state  it 
takes  up  carbon  from  the  fuel,  and  the  resulting  product  is 
highly  carbonized  iron. 

Fuel  for  the  Reduction  of  Iron  Ore. — The  carbon  fuel 
for  the  reduction  of  iron  ore  is  usually  supplied  in  the  form 
of  coke.  Formerly  most  of  the  coke  produced  in  this 
country  was  produced  by  the  distillation  of  bituminous  coal 
in  closed  retorts  known  as  "  bee-hive "  ovens,  in  which  all 
the  products  of  distillation  except  the  coke  were  wasted. 
At  the  present  time  about  60  per  cent,  of  the  coke  is  pro- 
duced in  ovens  designed  to  utilize  or  recover  the  products 
of  distillation  which  include  the  hydrocarbon  gases  and  the 
tar  and  ammonia  products  produced. 

In  some  pig-iron  producing  plants  anthracite  coal  is 
used  as  a  fuel.  In  a  few  furnaces  producing  a  very  high 
grade  of  pig  iron  charcoal  is  used.  Charcoal  contains 


THE  MANUFACTURE  OF  PIG  IRON  73 

fewer  contaminating  ingredients  than  does  coke,  but  is 
very  expensive.  Sulphur  is  an  ingredient  which  is  espe- 
cially troublesome  in  coke. 

Flux  Used  in  Reducing  Iron  Ore.— In  addition  to  ore  and 
fuel  it  is  necessary  in  the  reduction  of  iron  ore  to  provide 
some  material  which  will  unite  with  the  impurities  of  the 
ore  forming  a  fusible  mixture.  This  material  is  called  a 
flux  and  must  itself  be  free  from  undesirable  ingredients, 
especially  sulphur  and  phosphorus,  and  since  most  of  the 
impurities  found  in  iron  ore  give  an  acid  reaction  in  the 
reducing  process,  the  flux  should  be  strongly  basic.  Lime- 
stone is  the  flux  generally  used,  and  the  combination  formed 
of  flux  and  impurities,  known  as  slag,  is  fusible  and  lighter 
than  the  pig  iron  formed.  The  slag  floats  on  top  of  the 
molten  iron  and  this  renders  the  separation  of  iron  from 
slag  easy. 

The  Blast  Furnace. — The  reduction  of  iron  ore  is  carried 
out  in  a  tall  vertical  stack  lined  with  firebrick  known  as  a 
blast  furnace.  Fig.  25  is  a  diagram  of  a  blast  furnace  of  di- 
mensions common  in  American  practice.  The  names  of 
the  principal  parts  are  shown  in  Fig.  25  and  the  approxi- 
mate dimensions  can  be  determined  from  the  scale  at  the 
left  of  the  figure.  Fig.  26  is  from  a  photograph  of  a  blast- 
furnace plant  and  shows  the  general  arrangement,  with  the 
storage  piles  of  ore  and  limestone. 

The  ore,  the  fuel,  and  the  flux  for  the  blast  furnace  are 
carried  to  the  top  by  means  of  the  inclined  hoist  or  "skip" 
(see  Figs.  25  and  26)  and  there  are  fed  in  alternate  layers 
of  fuel,  ore,  and  flux  into  the  furnace  through  the  double 
bell  and  hopper  arrangement  which  is  so  designed  that  at  no 
time  is  the  interior  of  the  blast  furnace  in  direct  communi- 
cation with  the  outside  air.  The  *ore,  fuel,  and  flux  are 
fed  to  the  furnace  in  the  approximate  proportion  for  the 
whole  charge  of  one-sixth  flux,  a  little  less  than  one-third 
fuel,  and  a  little  more  than  one-half  ore.  The  combustion 
of  the  fuel  takes  place  principally  at  the  bottom  of  the 
furnace  in  a  blast  of  air  forced  through  nozzles  or  tuyeres 
under  a  pressure  of  about  15  Ib.  per  square  inch,  and  the 


MATERIALS  OF  ENGINEERING 


te 

g 

l( 

1  V 

I       I      I       I      I      I      I       I       I      I 
^SS 
ui   suoisuaujiQ  aftxuiycuddv  Jo^  ajcog 


THE  MANUFACTURE  OF  PIG  IRON 


75 


carbon  (coke)  is  changed  to  carbon  monoxide.  This  car- 
bon monoxide  coming  into  contact  with  the  hot  ore  reduces 
the  ore  to  molten  metallic  iron  which  drips  down  to  the 
hearth  at  the  bottom  of  the  furnace.  The  earthy  impuri- 
ties of  the  ore  unite  with  the  limestone  and  form  a  molten 
slag,  which  floats  on  top  of  the  molten  iron.  At  intervals 
the  slag  is  drawn  off  by  means  of  unplugging  the  cinder 
notch  and  the  iron  is  drawn  off  through  the  tap  hole  in  the 
hearth.  The  molten  iron  reduced  from  the  ore  as  it  drips 


Courtesy  of  Illinois  Steel  Co. 
FIG.  26. — General  view  of  blast-furnace  plant  U.  S.  Steel  Co.,  Gary,  Indiana. 


through  layers  of  partially  burned  coke  becomes  saturated 
with  carbon.  Some  of  the  carbon  absorbed  is  chemically 
combined  with  the  iron,  and  some  is  merely  mechanically 
mixed  with  the  iron  in  the  form  of  crystals  of  graphite. 
The  principal  chemical  changes  and  the  temperatures 
attained  are  shown  in  Fig.  27.  The  chemical  changes 
given  above  represent  in  a  general  way  what  happens 
in  a  blast  furnace.  The  actual  reactions  are  very 
complicated. 


76 


MATERIALS  OF  ENGINEERING 


Preheating  the  Blast,  Hot  Stoves. — The  blast  of  air 
blown  through  the  tuyeres  is  furnished  by  means  of  large 
blowing  engines.  A  great  increase  of  economy  of  operation 
of  the  furnace  is  brought  about  if  heated  air  is  blown  into 
the  furnace.  For  heating  the  incoming  air  the  heat  of  the 
exhaust  gases  of  the  blast  furnace  is  utilized  by  means  of 
a  " hot-blast  stove,"  shown  in  diagram  in  Fig.  25,  and  the 
utilization  of  heat  which  would  otherwise  be  wasted  is  a 


Smelting  Zone  Lime 

-forms  Slag  with 

earthy  Impur' 

fron  Melts. 

burns 

Manoxid 

-Tuyeres  ..- 


0  — 


Iron 


IniilmiliiiiliMiliiiilmil 

o         1000        eooo       5000 

Degrees   Fahrenheit 


FIG.  27. — Principal  reactions  and  temperatures  in  blast  furnace. 

source  of  great  economy  in  the  blast  furnace,  or,  for  that 
matter,  in  most  metallurgical  processes.  Each  stove  is  a 
steel  shell  lined  with  firebrick  and  filled  with  a  checker- 
work  of  firebrick  through  which  the  hot  gases  from  the 
blast  furnace  or  cold  air  from  outside  may  be  passed.  Four 
of  these  stoves  are  usually  used  with  a  single  blast  furnace. 
The  hot  gases  from  the  furnace  pass  through  three  of  these 
stoves  (stoves  2,  3  and  4).  The  waste  gases  contain  car- 


THE  MANUFACTURE  OF  PIG  IRON  77 

bon  monoxide,  an  inflammable  gas,  and  this  gas  is  burned 
as  it  passes  through  the  stoves,  and  the  brickwork  inside  the 
stove  is  heated  to  a  high  temperature.  Meanwhile  the 
outside  air  for  the  blast  is  being  forced  through  the  fourth 
stove  (stove  1,  Fig.  25)  which  has  been  previously  heated 
by  the-  escaping  gases  from  the  blast  furnace,  and  this  air 
is  heated  to  a  temperature  of  about  1,000°F.  before  being 
blown  through  the  tuyeres.  At  intervals  of  about  20  min. 
the  current  of  air  is  switched  from  one  stove  to  another. 
The  stoves  are  thus  alternately  heated  for  an  hour  by  the 
blast-furnace  gases  and  then  used  for  20  min.  to  heat  the 
incoming  air.  The  general  method  of  utilizing  the  heat  of 
exhaust  gases  to  preheat  the  air  for  a  furnace  is  called 
the  " regenerative"  process  and  is  widely  used  in  the  pro- 
duction of  metals.  Usually  there  is  more  heat  energy  in 
the  carbon  monoxide  of  the  blast  furnace  gases  than  is 
necessary  to  heat  the  blast,  and  the  excess  is  used  as  fuel 
under  the  boilers  for  the  blowing  engines  where  these  en- 
gines are  steam  engines,  or  is  used  directly  in  the  blowing 
engines  where  these  engines  are  gas  engines,  and  in  some 
cases  there  is  still  an  excess  of  carbon  monoxide  which  is 
utilized  in  driving  gas  engines  which  furnish  power  for 
general  consumption. 

In  a  blast  furnace  installation  the  escaping  gases  are 
led  through  dust  catchers  before  being  carried  through  the 
stoves  or  to  gas  engines.  These  dust  catchers  remove 
the  solid  particles  carried  along  by  the  waste  gas. 

Production  of  Pig  Iron. — The  operation  of  a  blast  fur- 
nace is  continuous.  A  blast  furnace  of  the  size  shown  in 
Fig.  25  is  capable  of  producing  about  500  tons  of  pig  iron 
per  day  of  24  hr.  To  produce  this  amount  of  pig  iron 
there  must  be  fed  into  the  furnace  about  1,000  tons  of  ore, 
500  tons  of  coke,  and  300  tons  of  limestone.  About  2,000 
tons  of  air  are  blown  through  the  furnace.  Molten  pig  iron 
is  drawn  off  about  every  6  hr. 

If  the  pig  iron  is  to  be  used  in  steel-making,  it  is  usually 
conveyed  from  the  blast  furnace  in  a  molten  condition  to 
the  steel  plant.  Special  ladle  cars  are  used  for  this  pur- 


78  MATERIALS  OF  ENGINEERING 

pose.  If  the  pig  iron  is  to  be  shipped  to  a  steel  plant  at  a 
distance  or  is  to  be  used  for  making  cast  iron,  it  is  cast 
into  small  pieces  about  2  ft.  long  weighing  about  100  Ib. 
each.  These  are  called  "pigs."  Two  methods  of  casting 
pigs  are  used.  The  older  method  uses  open  molds  in  a 
sand  bed  directly  in  front  of  the  blast  furnace.  The  molten 
pig  is  led  directly  through  the  sand  channels  to  the  pig 
molds.  This  method  is  simple,  but  the  heat  produced 
on  the  casting  floor  makes  the  work  of  manipulation  very 
exhausting,  and  considerable  time  elapses  after  a  melt  of 
iron  is  poured  into  the  molds  before  it  cools  sufficiently 
to  allow  the  removal  of  the  pigs,  and  the  preparation  of 
the  casting  bed  for  another  melt.  Sand  casting  beds 
for  pig  iron  are  not  much  used  today. 

A  more  recent  method  of  producing  pigs  of  iron  is  to 
use  casting  machines.  These  consist  of  cast-iron  molds 
lined  with  fireclay  and  arranged  as  buckets  on  an  endless 
chain.  These  buckets  are  passed  right  side  up  in  front 
of  the  blast  furnace,  and  are  filled  with  molten  pig  iron. 
The  chain  of  molds  is  long  enough  so  that  when  a  mold 
reaches  the  end  pulley  over  which  the  chain  turns,  the  pig 
iron  in  that  mold  has  solidified,  and  as  the  molds  are 
turned  upside  down  the  pig  is  dumped  into  a  car. 

Utilization  of  Blast-furnace  Slag. — The  slag  from  the 
blast  furnace  is  run  off  into  slag  cars  or  granulated  by 
contact  with  a  stream  of  water.  Blast-furnace  slag  is 
sometimes  utilized  in  making  Portland  cement,  and  in 
making  mineral  wool  (a  substance  used  for  heat  insulation 
or  "lagging").  It  is  also  sometimes  used  as  an  ingredient 
in  paint  or  as  ballast  material  for  railway  tracks. 

In  the  blast  furnace  iron  absorbs  other  impurities  be- 
sides carbon.  Sulphur  and  phosphorus  are  often  absorbed 
from  the  fuel  or  from  the  flux,  and  these  ingredients  are 
very  undesirable  in  iron  and  steel  because  they  cause  brittle- 
ness.  (In  iron  castings  in  which  brittleness  is  not  objec- 
tionable, such  as  stove  castings,  phosphorus  makes  the 
molten  cast  iron  very  fluid  and  the  castings  produced  very 
sharp  and  true  to  the  form  of  the  mold.)  Pig  iron  is  not 


THE  MANUFACTURE  OF  PIG  IRON  79 

used  directly  as  a  structural  material.  When  remelted  in 
a  foundry  cupola  with  scrap  iron  and  cast  into  molds  it  is 
known  as  cast  iron.  The  greatest  use  of  pig  iron  is  as  the 
raw  material  for  making  steel. 

Selected  References  for  Further  Study 

THURSTON:  "Text-book  of  the  Materials  of  Construction/'  New  York, 
1900,  Chaps.  II  and  III.  This  book  gives  an  especially  good  historical 
sketch  of  the  development  of  the  iron  industry. 

STOUGHTON:  "The  Metallurgy  of  Iron  and  Steel,"  New  York,  1911,  Chaps. 
I  and  II.  An  excellent  treatise  by  an  American  metallurgist. 

TURNER:  "The  Metallurgy  of  Iron,"  London,  1908,  Chaps.  I-X  inclusive. 
A  comprehensive  treatise  by  a  British  metallurgist. 

FORSYTHE:  "The  Blast  Furnace  and  the  Manufacture  of  Pig  Iron,"  New 
York,  1908.  A  concise  text  description  of  American  practice. 

MACPARLANE:  "The  Principles  and  Practice  of  Iron  and  Steel  Manufac- 
ture," London,  1906,  Chaps.  XVIII-XXIII  inclusive.  A  concise 
treatise  by  a  British  metallurgist. 


CHAPTER  VI 
THE  MANUFACTURE  OF  WROUGHT  IRON 

Importance  of  Wrought  Iron. — The  methods  in  use  for 
the  refining  of  pig  iron  all  include  the  removal  of  carbon 
from  the  pig  iron  by  means  of  oxidation.  The  oldest 
method  of  refining  pig  iron  is  the  puddling  process  which 
produces  wrought  iron. 

Up  to  the  latter  part  of  the  nineteenth  century  wrought 
iron  was  the  most  important  of  the  iron  and  steel  products, 
but  the  development  of  the  Bessemer  and  of  the  open- 
hearth  processes  for  refining  pig  iron  into  steel  made 
wrought  iron  of  secondary  importance  as  a  structural 
material.  Wrought  iron  is,  however,  still  used  extensively 
for  general  blacksmithing  work,  on  account  of  the  ease  of 
welding  wrought  iron  as  compared  with  steel;  and  for 
water  and  gas  pipes  on  account  of  the  judgment  of  many 
users  that  wrought  iron  rusts  less  easily  than  does  steel 
(upon  this  question  there  exist  sharp  differences  of  opinion 
among  metallurgists,  see  page  149).  Wrought  iron  is  also 
used  for  bolts  and  rods  subjected  to  severe  shock  on 
account  of  the  belief  held  by  some  engineers  that  wrought 
iron,  on  account  of  its  "  fibrous "  nature,  offers  better 
resistance  to  shock  than  does  steel  (see  page  38) . 

Definition  of  Wrought  Iron. — Whether  wrought  iron 
shall  be  distinguished  from  steel  by  its  low  carbon  content 
or  by  its  method  of  manufacture  is  a  much-debated  ques- 
tion. The  following  definition  as  given  by  the  American 
metallurgist  Bradley  Stoughton  has  been  adopted  for  this 
book: 

"  Wrought  iron  is  almost  the  same  as  the  very  low-carbon  steels  except 
that  it  is  never  produced  by  melting  and  casting  in  a  mold  but  is  always 
forged  to  the  desired  size  and  form.  It  usually  contains  less  than  0.12 
per  cent,  of  carbon.  Its  chief  distinction  from  the  low-carbon  steels  is 

80 


THE  MANUFACTURE  OF  WROUGHT  IRON        81 

that  it  is  made  by  a  process  which  finishes  it  in  a  pasty,  instead  of  a 
liquid  form  and  leaves  about  1  to  2  per  cent,  of  slag  mechanically 
disseminated  through  it." 

% 

The  Puddling  Process. — Wrought  iron  is  produced  by 
the  removal  of  impurities  from  pig  iron  in  a  furnace  known 
as  a  puddling  furnace.  A  special  pig  iron  known  as  forge 
pig  is  used.  Forge  pig  contains  a  high  percentage  of  silicon 
which  aids  in  forming  a  layer  of  molten  slag  protecting  the 
molten  pig  iron  from  oxidation  by  the  air.  A  sectional 
diagram  of  a  puddling  furnace  is  shown  in  Fig.  28.  The 
fuel  usually  used  is  soft  coal  (occasionally  gas  is  used)  and 


FIG.  28. — Diagram  of  puddling  furnace. 

the  fire  is  built  on  the  grate  G;  the  flame  passes  over  the  fire 
bridge  and  is  reflected  by  the  roof  of  the  furnace  to  the 
hearth,  which  is  bedded  or  "fettled"  with  rich  iron  oxide 
which  has  a  basic  reaction,  offsetting  the  acid  properties 
of  the  silicon  in  the  forge  pig.  A  charge  of  about  500  Ib. 
of  forge  pig  is  placed  on  the  hearth  and  the  draft  to  the 
chimney  is  opened  wide  so  that  the  fire  burns  vigorously. 
In  about  half  an  hour  the  pig  iron  is  melted.  During  this 
melting  stage  nearly  all  the  silicon,  manganese,  and  a  con- 
siderable part  of  the  sulphur  and  phosphorus  present  have 
been  taken  from  the  pig  by  uniting  with  the  basic  iron  oxide 
to  form  a  slag.  It  is  necessary  that  the  slag  should  be 
basic,  else  the  phosphorus  will  return  to  the  iron. 


82  MATERIALS  OF  ENGINEERING 

After  the  melting  stage  is  complete  the  damper  in  the 
stack  is  closed  and  the  temperature  of  the  furnace  reduced. 
There  ensues  a  combination  of  the  carbon  of  the  pig  with 
the  oxygen  of  the  iron  oxide  and  carbon  monoxide  (CO) 
is  formed.  This  stage  of  the  puddling  process  is  called  the 
"bony  on  account  of  the  frothing  and  increase  of  volume 
of  the  molten  charge  in  the  furnace  as  the  carbon  monoxide 
bubbles  through  it.  The  charge  rises  in  the  furnace  much 
as  boiling  molasses  rises  in  a  kettle,  and  this  rising  permits 
the  removal  of  about  half  the  slag,  by  allowing  it  to  "boil 
over"  the  sill  of  the  slag  door  S  into  a  ladle  called  a  "slag 
buggy"  placed  just  outside  the  slag  door.  During  the 
boil  particles  of  pasty  iron  begin  to  separate  from  the  boil- 
ing mixture  of  molten  slag  and  molten  pig  iron,  and  by 
opening  the  chimney  damper  the  temperature  is  gradually 
raised  to  preserve  a  proper  degree  of  fluidity  of  the  charge. 
The  furnace  operator,  known  as  the  "puddler, "  prevents 
these  particles  of  iron  from  sticking  to  the  bottom  of  the 
furnace  by  vigorously  stirring  ("rabbling")  the  charge 
with  a  long  iron  hoe  through  the  door  D. 

After  the  boil  has  been  in  progress  for  about  half  an  hour 
all  the  iron  has  separated  in  pasty  masses  from  the  sur- 
rounding slag  and  is  said  to  "come  to  nature."  The 
temperature  of  the  furnace  is  reduced  by  closing  the 
damper  in  the  chimney,  and  the  puddler  and  his  assistant 
by  means  of  a  long  rod  gather  the  pasty  particles  of  iron 
into  "muck"  balls  weighing  about  100  Ib.  apiece,  and  these 
balls  (four  or  five  for  each  heat)  are  removed  one  by  one 
from  the  furnace  (see  Fig.  29).  These  balls  contain  a  large 
amount  of  slag  mixed  with  the  iron.  They  are  removed 
to  a  squeezer  or  a  hammer  where  most  of  the  liquid  slag 
is  squeezed  out  of  the  ball,  much  as  water  is  squeezed  out 
of  a  sponge.  After  squeezing,  the  balls  are  rolled  into 
bars  known  as  "muck  bars;"  these  muck  bars  are  cut 
and  piled  in  crosswise  layers,  heated  to  a  welding  heat  and 
rolled  into  "merchant"  bars, — flats,  rounds,  squares,  plate, 
or  other  shapes.  This  "merchant  bar"  is  the  wrought 
iron  of  commerce. 


THE  MANUFACTURE  OF  WROUGHT  IRON        83 

Characteristics  of  Wrought  Iron. — The  distinguishing 
characteristic  of  wrought  iron  is  the  presence  of  fibers  of 
slag  extending  through  the  iron  in  the  direction  of  rolling. 
Slag  is  never  completely  removed  by  the  squeezing  process. 


Charging  the  puddling  furnace. 


Courtesy  of  Interstate  Iron  and  Steel  Co. 

Preparing  to  draw  the  ball. 

FIG.  29. — View   in   puddling-furnace   plant.     Interstate    Iron   and    Steel    Co., 

Chicago,  111. 

The  characteristic  slag  fibers  can  usually  be  detected  by 
etching  a  polished  surface  with  acid.  If  the  material  is 
wrought  iron  the  slag  fibers  cause  the  surface  to  turn  black. 


84  MATERIALS  OF  ENGINEERING 

These  slag  fibers  can  be  detected  with  certainty  under  the 
microscope  (see  Fig.  30).  Small  bars  of  wrought  iron  may 
sometimes  be  distinguished  from  bars  of  soft  steel  by  allow- 
ing both  to  fall  on  a  stone  or  on  a  concrete  floor;  the  steel 
"rings"  while  the  iron  gives  a  dull  thud.  Another  test  is 
the  "nick  bend"  test.  A  bar  of  the  material  is  nicked  with 
a  sharp  chisel  and  broken  by  bending.  Wrought  iron 
shows  a  fibrous  fracture  for  such  a  test.  A  careful  exami- 
nation for  the  presence  of  slag  is,  however,  the  only  certain 
means  of  distinguishing  wrought  iron  from  steel. 


FIG.  30. — Longitudinal  section  of  wrought  iron  showing  slag  "fiber."     Photo- 
micrograph by  E.  O.  Dixon  and  Jos.  Simons.     Magnification  350  times. 

Wrought  iron  is  more  costly  than  low-carbon  Bessemer 
or  open-hearth  steel,  and  this  leads  to  the  occasional  adul- 
teration of  wrought  iron  with  a  cheap  grade  of  scrap  steel. 
A  mixture  of  bars  of  wrought  iron  and  of  scrap  steel  is  piled 
together,  heated  to  a  welding  temperature,  rolled  into 
"merchant  bars"  and  sold  as  wrought  iron.  Of  course, 
this  mixture  is  not  wrought  iron,  and  it  exhibits  many  of 
the  properties  of  soft  steel.  This  adulterated  wrought  iron 
should  be  distinguished  from  the  "knobbled"  wrought 
iron  described  in  a  succeeding  paragraph. 

Chemical  Composition  of  Wrought  Iron.  —The  chemical 
composition  of  different  grades  of  wrought  iron  commonly 
found  on  the  market  is  given,  approximately,  by  the  fol- 
lowing table,  the  values  being  expressed  in  per  cent. 


THE  MANUFACTURE  OF  WROUGHT  IRON 


85 


Carbon 

,    Phos- 
phorus 

Sulphur 

Silicon 

Man- 
ganese 

Slag 

Common  wrought  iron  .  . 

0.08 

0.25 

0.05 

0.21 

0.10 

3.00 

"Best"  wrought  iron.  .  .  . 

0.06 

0.15 

0.03 

0.20 

0.06 

2.80 

Swedish  wrought  iron  .  .  . 

0.05 

0.055 

0.007  1  0.015 

0  006 

0.61 

!                1                ' 

Charcoal  Iron. — While  a  mixture  of  steel  scrap  and 
wrought  iron  merely  welded  together  is  a  very  inferior 
product,  an  excellent  grade  of  wrought  iron  is  made  from 
steel  scrap  by  melting  or  " sinking"  in  a  small  puddling  or 
"knobbling"  furnace  in  which  charcoal  is  used  as  a  fuel. 
A  highly  basic  slag  is  used  as  in  a  large  puddling  furnace, 
and  the  entire  charge  of  steel  scrap  is  transformed  into 
true 'wrought  iron.  The  removal  of  impurities  is  very 
nearly  complete.  Charcoal  iron  is  used  for  some  electrical 
apparatus,  and  for  boiler  tubes  where  a  high  degree  of 
purity  seems  to  give  to  the  iron  a  power  of  resistance  to 
corrosion. 

Selected  References  for  Further  Study 

THURSTON:  "Text-book  of  the  Materials  of  Construction,"  New  York, 
1900,  Chap.  IV.  Especially  valuable  for  the  historical  data  it  contains. 
Not  descriptive  of  recent  practice. 

STOUGHTON:  "The  Metallurgy  of  Iron  and  Steel,"  New  York,  1911,  Chaps. 
Ill  and  IV.  An  excellent  general  treatise  by  an  American  metallurgist. 

TURNER:  "The  Metallurgy  of  Iron,"  London,  1908,  Chaps.  XIII-XVI  in- 
clusive. A  comprehensive  treatise  by  a  British  metallurgist. 

CAMPBELL:  "The  Manufacture  and  Properties  of  Iron  and  Steel,"  New 
York,  1907,  Chap.  III.  An  excellent  general  treatise,  somewhat  tech- 
nical, by  an  American  metallurgist. 

MACFARLANE:  "The  Principles  and  Practice  of  Iron  and  Steel  Manufac- 
ture," London,  1906,  Chap.  III.     A  concise  text  by  a  British  metal- 
lurgist. 
For  references  on  the  relative  durability  of  wrought  iron  and  steel  see 

list  at  the  end  of  Chap.  XIIL 


CHAPTER  VII 
THE  MANUFACTURE  OF  OPEN-HEARTH  STEEL 

General  Features. — In  the  refining  of  pig  iron  into  steel 
complete  fusion  takes  place.  The  steel  is  drawn  off  from 
the  refining  furnace  in  a  molten  state  instead  of  being  re- 
moved in  pasty  balls  as  is  the  case  with  wrought  iron.  The 
process  most  extensively  used  in  making  steel  is  the  open- 
hearth  process,  sometimes  called  the  Siemens-Martin  proc- 
ess. In  this  process  .pig  iron,  steel  scrap,  and  iron  ore  are 
placed  on  a  shallow  hearth,  and  are  subjected  to  intense 
heat  from  a  flame  of  gas  or  liquid  fuel.  The  pig  iron,  steel 
scrap,  and  iron  ore  constitute  the  charge,  and  under  the  in- 
tense heat  the  charge  melts,  and  a  slag  is  formed  which 
floats  on  top  of  the  melted  charge.  This  slag  contains  the 
iron  ore  (iron  oxide),  and  the  carbon  and  other  impurities  of 
the  pig  iron  are  oxidized  and  go  into  the  slag.  Direct  con- 
tact of  air  with  the  molten  pig  iron  would  oxidize  not  only 
the  impurities,  but  a  considerable  amount  of  iron  as  well. 
The  oxygen  in  the  slag  is  in  the  form  of  iron  oxide,  which 
will  not  attack  iron,  but  will  oxidize  the  impurities  in  the  pig 
iron.  Air,  which  is  fed  to  the  furnace  with  the  fuel,  oxidizes 
the  slag,  and  the  slag  not  only  oxidizes  the  impurities  in  the 
pig  iron,  but  also  serves  as  a  protective  blanket  against 
direct  action  of  the  air  on  the  molten  pig  iron.  The  iron 
oxide  in  the  slag  serves  as  a  carrier  of  oxygen  from  the  air 
to  the  impurities  in  the  pig  iron. 

Basic  and  Acid-steel  Processes. — In  most  iron  ores  phos- 
phorus is  present  in  small  quantities.  Phosphorus  is  a  very 
undesirable  ingredient  in  steel,  because  it  makes  steel  brittle, 
and  if  ores  containing  considerable  phosphorus  are  to  be 
used — and  the  supply  of  low-phosphorus  ores  is  rapidly 
diminishing — this  phosphorus  must  be  removed  in  the  proc- 
ess for  refining  pig  iron  into  steel.  Phosphorus  oxidizes 

86 


THE  MANUFACTURE  OF  OPEN-HEARTH  STEEL  87 

readily  under  the  action  of  the  slag  in  the  open-hearth  proc- 
ess, but  if  the  slag  is  acid,  the  phosphorus  is  found  com- 
bined with  the  steel  again  as  the  steel  is  drawn  out  of  the 
furnace.  If,  however,  the  slag  is  basic,  the  phosphorus 
combines  with  the  slag,  and  the  steel  is  freed  from  its  ob- 
jectionable presence. 

In  the  steel-making  process  in  widest  use  in  the  United 
States  calcined  limestone  is  added  to  the  charge  fed  into  the 
furnace.  This  renders  the  reaction  of  the  molten  charge 
basic,  and  the  phosphorus  in  the  pig  iron  and  in  the  ore  of 
the  charge  is  removed  from  the  steel  and  carried  into  the 
slag,  where  it  remains.  In  the  acid  process,  which  is  less 
used,  no  limestone  is  added  to  the  charge,  the  reaction  of  the 
molten  charge  is  acid,  and  the  phosphorus  of  the  pig  remains 
in  the  steel.  The  addition  of  the  limestone  and  the  removal 
of  phosphorus  renders  the  basic  open-hearth  process  some- 
what more  laborious  and  somewhat  longer  than  is  the  acid 
process,  but  the  cost  of  the  extra  labor  is  more  than  com- 
pensated by  the  ability  to  make  use  of  the  cheaper  ores  of 
iron,  which,  in  this  country  are  too  high  in  phosphorus  for 
the  acid  process. 

The  Open-hearth  Furnace.— Figs.  31,  32  and  33  show 
diagrams  and  a  general  view  of  the  type  of  open-hearth 
furnaces  common  in  the  United  States.  A  shallow  hearth 
H  lined  with  refractory  brick  contains  the  charge,  and 
above  this  hearth  is  a  roof  lined  with  firebrick.  Gas  is  the 
fuel  commonly  used.  Gas  and  air,  preheated,  are  admitted 
through  ports,  P.  A  chimney  provides  draft.  The  ingre- 
dients of  the  charge  (pig  iron,  steel  scrap,  iron  ore,  and,  for 
the  basic  process  calcined  limestone)  are  fed  through  a 
charging  door  D,  and  after  the  process  is  complete  the 
melted  steel  is  tapped  off  through  a  tap  hole  T. 

The  air  is  preheated  by  passing  it  through  passages  C 
which  are  filled  with  a  checkerwork  of  brick  which  has  been 
preheated  by  the  exhaust  gases  from  the  furnace.  The 
regenerative  principle  is  the  same  as  that  utilized  in  the 
' ' stoves  "  of  the  blast  furnace.  At  least  four  heating  cham- 
bers are  used  with  each  open-hearth  furnace.  As  shown 


88 


MATERIALS  OF  ENGINEERING 


in  Fig.  31  they  are  located  under  the  furnace,  and  the  cur- 
rents of  incoming  air  and  gas  and  outgoing  hot  gas  are 


Scale  of  Approximate  Dimensions  in  Ft 
K)         20         50 


FIG.  31. -^Diagram  of  open-hearth  steel  furnace. 


Scale  of  Approximate  Dimensions  in  ft 
0  10          ZO  30 


FIG.  32.- — Diagram  of  open-hearth  steel  plant.     Section  of  furnace  is  at  right 
angles  to  the  section  shown  in  Fig.  31. 

switched  from  one  chamber  to  another  at  intervals  of  about 
20  min. 


THE  MANUFACTURE  OF  OPEN-HEARTH  STEEL  89 

The  general  appearance  of  acid  open-hearth  furnaces  and 
basic  open-hearth  furnaces  is  the  same.  Fig.  33  is  from 
photographs  of  an  open-hearth  plant.  An  acid  open- 
hearth  furnace  is  lined  with  some  form  of  silica  brick,  and 
a  basic  furnace  with  some  form  of  dolomitic  limestone,  or 
with  calcined  magnesite.  The  silica  brick  has  an  acid 
reaction,  and  resists  corroding  action  by  the  charge  fed 
to  the  furnace  in  the  acid  process ;  the  dolomitic  limestone 
has  a  basic  reaction,  and  resists  corroding  action  by  the 
charge  fed  to  the  furnace  in  the  basic  process.  The  lin- 
ing of  an  openhearth  furnace  takes  no  active  part  in  the 
refining  of  the  charge  into  steel. 

Charging  the  Open -hearth  Furnace. — The  charge  of  pig 
iron,  steel  scrap,  iron  ore  and,  for  basic  open-hearth  fur- 
naces, limestone  is  sometimes  fed  to  the  furnace  by  hand, 
but  more  often  by  a  charging  machine.  A  common  type 
of  charging  machine  is  shown  at  M  in  Fig.  32.  The  opera- 
tor seated  at  the  rear  end  of  the  machine  0,  a  safe  distance 
away  from  the  heat  of  the  furnace,  can  control  the  motor- 
driven  mechanism  which  picks  up  the  bucket  B  containing 
the  charge,  which  shoves  the  bucket  with  its  charge  into 
the  furnace  (broken  lines  in  Fig.  32),  which  dumps  the 
charge,  and  which  backs  the  empty  bucket  out  of  the  fur- 
nace. The  pig  iron  of  the  charge  is  fed  to  the  furnace, 
sometimes  in  a  solid  state,  but  usually  molten.  Fig.  33a 
shows  molten  pig  iron  being  fed  into  the  furnace. 

The  Control  of  the  Open-hearth  Process. — The  open- 
hearth  furnace  shown  in  Figs.  31  and  32  has  a  capacity 
of  about  60  tons  per  charge,  which  is  about  the  average 
size  for  open-hearth  furnaces.  About  8  hr.  are  required 
to  refine  a  charge.  The  process  may  be  watched  through 
peep  holes  in  the  furnace  doors,  and  at  intervals  small 
samples  of  the  molten  contents  of  the  furnace  are  dipped 
out,  run  into  molds,  cooled  and  broken.  Whether  the 
refining  process  has  progressed  far  enough  may  be  told  by 
the  appearance  of  the  fracture  of  these  test  pieces,  of  from 
a  quick  chemical  analysis.  If  from  the  condition  of  a  test 
sample  it  is  found  necessary  to  oxidize  out  more  impurities, 


90  MATERIALS  OF  ENGINEERING 

iron  ore  may  be  added  to  the  charge  in  the  furnace.  The 
open-hearth  process  is  under  excellent  control. 

Recarburization  of  Steel. — Usually  in  the  open-hearth 
process,  especially  in  the  basic  open-hearth  process,  in 
order  to  insure  the  thorough  removal  of  impurities  the 
refining  process  is  carried  so  far  that  the  product  is  lower 
in  carbon  than  is  desired.  This  is  remedied  by  adding 
carbon  in  some  form,  usually  in  the  form  of  ferromanganese, 
a  pig  iron  rich  in  manganese  and  carbon;  sometimes  the 
addition  of  carbon  is  made  in  the  form  of  powdered  coke 
or  charcoal.  This  addition  of  carbon  to  the  refined  charge 
is  called  recarburization.  Recarburization  for  acid  open- 
hearth  steel  is  sometimes  carried  out  in  the  furnace  before 
tapping  the  charge,  but  for  basic  steel  recarburization  is 
carried  out  in  the  ladle  into  which  the  charge  is  tapped  and 
from  which  the  slag  has  been  removed.  The  manganese 
of  the  ferromanganese  used  as  a  recarburizer  is  a  valuable 
ingredient  to  add  to  steel  on  account  of  its  powerful  affinity 
for  oxygen  and  sulphur.  Its  addition  tends  to  cause  any 
oxygen  present  to  combine  with  the  manganese  rather  than 
with  the  iron  and  also  serves  to  neutralize  the  bad  effects 
of  any  sulphur  present  by  causing  a  combination  of  sul- 
phur and  manganese  in  place  of  a  combination  of  sulphur 
and  iron.  Sulphur  is  an  undesirable  element  in  steel, 
making  it  brittle  when  hot,  and  therefore  very  difficult  to 
forge  or  roll  into  shape.  Recently  the  open-hearth  process 
has  been  successfully  used  to  produce  a  product  very  low 
in  carbon  and  manganese — chemically,  almost  pure  iron. 
This  product  is  claimed  to  offer  great  resistance  to  corrosion. 

Other  Types  of  the  Open-hearth  Furnace. — The  type 
of  the  open-hearth  furnace  shown  in  Fig.  33,  called  the 
stationary  type,  is  the  one  most  commonly  used.  The 
tapping  of  the  furnace  is  accomplished  by  piecing  a  clay 
plug  in  the  tap  hole;  this  is  inconvenient,  and  sometimes 
at  a  critical  stage  of  the  process  delay  is  caused  in  doing  this. 
Another  form  of  open-hearth  furnace  is  the  tilting  furnace. 
In  this  form  of  furnace  the  steel  is  discharged  by  tipping  the 
whole  furnace,  which  is  mounted  on  rockers,  until  the  tap 


THE  MANUFACTURE  OF  OPEN-HEARTH  STEEL     91 


Courtesy  of  Illinois  Steel  Co,  fc.   Pouring  side. 

FIG.  33. — General  view  in  open-hearth  steel  plant  U.  S.  Steel  Co.,  Gary,  Indiana. 


92  MATERIALS  OF  ENGINEERING 

hole  comes  below  the  level  of  the  molten  charge.  The  tilt- 
ing furnace  is  very  satisfactory  in  operation  but  its  first 
cost  and  the  cost  of  repairs,  power  for  tilting,  and  general 
upkeep  are  higher  than  for  the  stationary  furnace. 

Fuel  for  the  Open-hearth  Furnace. — The  fuel  in  common 
use  in  open-hearth  steel  furnaces  is  producer  gas.  Pro- 
ducer gas  is  generated  in  an  apparatus  called  a  producer 
by  passing  air  mixed  with  steam  through  incandescent 
coal.  Producer  gas  is  a  very  cheap  gas  of  low  heating 
power.  Other  fuels  used  in  open-hearth  furnaces  are 
natural  gas  (where  available)  and  oil  in  the  form  of  spray. 
Powdered  coal  and  tar  have  each  been  used  to  a  limited 
extent,  and  gives  promise  of  usefulness  as  fuels  for  open- 
hearth  furnaces. 

Arrangement  of  Open-hearth  Steel  Plants. — In  an  open- 
hearth  steel  plant  a  number  of  open-hearth  furnaces  are 
placed  end  to  end,  and  this  row  of  furnaces  is  served  from 
a  charging  floor  in  front  of  the  furnace  by  one  or  more 
charging  machines.  The  steel  is  tapped  from  the  rear  of 
the  furnaces  on  a  casting  floor  situated  at  a  lower  level 
than  the  charging  floor.  Fig  32  indicates  the  arrangement 
of  a  plant  in  diagram,  and  Fig.  33  shows  views  of  the 
charging  floor  and  of  the  casting  floor  of  an  open-hearth 
steel  plant.  On  the  casting  floor  the  steel  is  tapped  into 
a  ladle  L  which  carries  it  to  molds.  If  steel  castings  are 
to  be  made,  these  molds  are  shaped  so  as  to  produce  the 
forms  desired;  if  rolled-steel  rods,  plates,  or  shapes  are  to 
be  the  product  the  molds  are  called  ingot  molds  and  the 
steel  is  produced  in  large  blocks  or  ingots  which  are  rolled 
into  the  shapes  desired.  The  production  of  steel  castings 
is  treated  in  Chap.  X  and  the  production  of  rolled  steel 
in  Chap.  XI. 

Uses  and  Limitations  of  Open-hearth  Steel. — The 
product  of  the  open-hearth  furnace  varies  in  chemical 
composition  from  almost  pure  iron  up  to  steel  with  1  per 
cent,  of  carbon.  Open-hearth  steel  is  used  for  making 
steel  castings,  and  for  making  rails,  rods,  plates,  structural 
shapes,  and  spring  steel.  Owing  to  the  greater  purity  of 


THE  MANUFACTURE  OF  OPEN-HEARTH  STEEL  93. 

the  pig  iron  which  must  be  used  in  the  acid  open-hearth 
process,  acid  open-hearth  steel  is  more  costly  than  basic 
open-hearth  steel,  and  by  some  users  is  considered  to  be  of 
higher  quality. 

The  refining  action  of  the  open-hearth  process  is  limited 
by  the  fact  that  air  is  blown  through  the  furnace  with  the 
fuel,  and  that  if  the  removal  of  impurities  is  carried  to  the 
extreme,  the  steel  itself  becomes  seriously  oxidized.  At 
the  present  time  for  the  very  highest  grades  of  steel  it  is 
necessary  to  use  processes  which  refine  steel  without  direct 
contact  of  air  with  the  charge  in  the  steel  furnace,  such  as 
the  crucible  process  and  the  electric  furnace  process.  How- 
ever, for  all  but  the  highest  grades  of  steel,  the  open- 
hearth  process  gives  very  satisfactory  results. 

Selected  References  for  Further  Study 

STOUGHTON:  "The  Metallurgy  of  Iron  and  Steel,"  New  York,  1911,  Chaps. 

Ill  and  VI.     An  excellent  general  treatise  by  an  American  metallurgist. 
HARBORD    AND    HALL:   "The    Metallurgy  of  Steel,"  London,   1916,   Vol. 

Chaps.  VI-VIII  inclusive.     A  comprehensive  treatise  by  two  British 

metallurgists. 
CAMPBELL:  "The  Manufacture  and  Properties  of  Iron  and  Steel,"  New 

York,  1907,  Chaps.  VIII-XII  inclusive.     An  excellent  general  treatise, 

somewhat  technical,  by  an  American  metallurgist. 

MACFARLANE:  "The  Principles  and  Practice  of  Iron  and  Steel  Manufac- 
ture," London,  1906,  Chaps.  X-X1I  inclusive.     A  concise  text  by  a 

British  metallurgist. 


CHAPTER  VIII 

THE  MANUFACTURE  OF  STEEL  BY  THE  BESSEMER 

PROCESS 

General  Features. — Next  to  the  open-hearth  process  the 
most  widely  used  process  for  refining  pig  iron  is  the  Besse- 
mer process,  so  named  from  its  inventor,  Sir  Henry  Besse- 
mer. Until  recently  this  was  the  most  important  process 
for  making  steel.  In  the  Bessemer  process  molten  pig  iron 
is  poured  into  a  pear-shaped  vessel,  called  a  converter,  cold 
air  is  blown  through  the  molten  pig  iron,  and  the  oxygen 
of  the  air  burns  out  practically  all  the  impurities  of  the  pig 
iron,  including  carbon;  there  is  left  very  nearly  pure  iron. 
To  this  molten  iron  carbon  is  added  in  the  proportions  de- 
sired in  the  finished  product.  In  this  process  no  outside 
fuel  is  used,  the  impurities  in  the  pig  iron  furnish  fuel  for  the 
process. 

The  Bessemer  Converter. — The  purification  of  pig  iron 
is  carried  on,  in  the  Bessemer  process,  in  a  pear-shaped 
vessel  called  the  converter.  Fig.  34  shows  in  diagram  a 
section  of  a  converter,  and  Fig.  35  is  from  a  photograph  of 
a  battery  of  converters  in  action.  The  usual  capacity  of  a 
Bessemer  converter  is  from  10  to  20  tons  of  molten  metal 
per  charge. 

The  converter  is  lined  with  bricks  of  refractory  material, 
and  the  bottom  is  pierced  with  holes  or  tuyeres.  Through 
these  holes  air  is  forced  under  a  pressure  of  about  20  Ib.  per 
square  inch,  the  necessary  air  pressure  being  supplied  by 
means  of  blowing  engines.  As  shown  in  Figs.  34  and  35, 
the  converter  is  mounted  on  trunnions;  one  of  the  trun- 
nions is  hollow  and  serves  as  the  entrance  pipe  for  the  air 
blast. 

Pig  Iron  for  the  Bessemer  Process. — The  molten  pig 
ron  which  serves  as  raw  material  for  the  Bessemer  process 

94 


MANUFACTURE  OF  STEEL 


95 


is  the  product  of  several  blast  furnaces.  It  is  carried 
molten  in  ladle  cars  to  a  large  receiving  vessel  called  a  mixer. 
From  this  mixer  the  pig,  still  molten,  is  carried  to  the  Besse- 
mer converter.  The  object  of  the  mixer  is  to  secure  pig 
iron  with  the  proper  composition,  and  of  a  higher  degree  of 
uniformity  than  could  be  secured  from  one  blast  furnace 


MOLTEN  J  IRQN-.  ~.-~ 


Scale  of  Approximate  Dimensions  in  F-h 
FIG.  34. — Diagram  of  Bessemer  converter. 

alone.  Bessemer  pig  is  the  name  given  to  pig  iron  suitable 
for  conversion  to  Bessemer  steel.  It  contains  about  3 
to  4  per  cent,  of  carbon,  1  to  1.5  per  cent,  of  silicon,  less 
than  0.1  per  cent,  of  phosphorus,  and  only  small  quantities 
of  sulphur  and  manganese. 

The  Operation  of  the  Bessemer  Converter. — The  general 
arrangement  of  a  Bessemer  steel  plant  is  shown,  in  diagram, 


96  MATERIALS  OF  ENGINEERING 

by  Fig.  36.  To  receive  the  charge  of  molten  pig  iron  the 
converter  is  tipped  down  to  the  position  shown  in  the  solid 
lines  and  the  molten  pig  is  poured  in.  The  air  blast  is 
then  turned  on  and  the  converter  turned  to  an  upright  posi- 
tion (dotted  lines  Fig.  36).  The  "blow"  is  then  said  to  be 
in  progress.  Under  the  heat  of  the  molten  iron  the  im- 
purities in  it  ignite  and  burn  in  the  current  of  air  forced 
through  the  molten  mass,  and  the  temperature  is  increased. 


Courtesy  of  Illinois  Steel  Co. 

FIG.  35. — Battery  of  Bessemer  converters  in  action,  Illinois  Steel  Co.,  South 
Chicago,  111.  Converter  A  and  converter  C  are  "blowing."  Steel  from  con- 
verter B  is  being  poured  into  ladle  of  hydraulic  crane  at  D.  Steel  is  being  poured 
into  ingot  molds  at  E. 

Silicon  and  manganese  are  the  first  ingredients  to  burn  out; 
they  burn  with  a  yellow  flame.  After  about  4  min.  carbon 
begins  to  burn  out  with  an  intense  white  flame.  After  the 
carbon  is  burned  out,  which  occurs  after  about  10  min.  of 
"blow,"  the  flame  drops  and  the  contents  of  the  converter 
have  become,  chemically,  nearly  pure  iron  containing,  how- 
ever some  iron  oxide  and  absorbed  gases.  Attempts  to 
use  this  iron  from  the  Bessemer  converter  have  shown  it 


MANUFACTURE  OF  STEEL 


97 


to  be  weak  and  brittle  on  account  of  the  presence  of  ab- 
sorbed gases  and  of  iron  oxide,  and  it  has  been  found  neces- 
sary to  add  manganese  to  combine  with  the  oxygen  of  the 
iron  oxide  removing  it  from  the  metal. 

When  the  flame  of  the  converter  " drops"  the  converter 
is  tipped  into  pouring  position  (dot  and  dash  lines,  Fig.  36), 
the  blast  of  air  is  shut  off,  and  the  mass  of  molten  iron  is 
poured  into  a  ladle.  At  the  same  time  there  is  poured  into 
the  ladle  a  small  quantity  of  molten  pig  iron  high  in  carbon 


20- 


Control  Levers  -for 
r— |  Tipping  Converter        CAR 
H  and  Operating  Crane 


FIG.  36. — Diagram  of  Bessemer-steel  plant. 

and  manganese.  If  the  final  product  of  the  Bessemer  proc- 
ess is  to  be  low-carbon  steel,  this  recarburizing  pig  iron  is 
usually  ferromanganese,  if  the  product  is  to  be  high-carbon 
steel  a  pig  iron  known  as  spiegeleisen  is  generally  used. 
The  ferromanganese  or  the  spiegeleisen  is  melted  in  a  sepa- 
rate cupola,  and  is  brought  in  a  molten  state  to  the  Besse- 
mer plant.  The  manganese  in  the  recarburizer  acts  very 
effectively  to  absorb  any  oxygen  present  in  the  iron  and  thus 
to  prevent  this  oxygen  uniting  with  the  iron.  Manganese 
also  unites  with  sulphur,  forming  manganese  sulphide, 


98  MATERIALS  OF  ENGINEERING 

which  is  less  injurious  to  the  steel  than  is  iron  sulphide. 
The  carbon  of  the  recarburizer  unites  with  the  iron,  and 
steel  is  the  resulting  product. 

After  the  recarburizing  process  is  complete,  the  ladle  of 
molten  steel  is  lifted  by  a  hydraulic  crane  over  a  row  of 
ingot  molds,  which  are  successively  filled  with  the  molten 
steel,  as  shown  in  Figs.  35  and  36. 

Basic  Bessemer  Process. — The  Bessener  process  as 
outlined  above  is  that  common  in  the  Bessemer  plants  of 
the  United  States.  No  elimination  of  phosphorus  or 
sulphur  takes  place  in  the  process,  and  the  quality  of  the 
steel  produced  depends  largely  on  the  quality  of  pig  iron 
used  as  raw  material. 

In  England  and  especially  in  Germany,  a  basic  Bessemer 
process  is  widely  used.  In  the  basic  Bessemer  process, 
limestone  is  added  to  the  charge,  and  the  blow  is  continued 
for  several  minutes  after  the  carbon  is  burned  out.  The 
addition  of  the  limestone  causes  a  basic  slag  to  form,  and 
the  phosphorus  of  the  pig  iron  is  oxidized  and  retained  as 
calcium  phosphate  by  this  basic  slag.  The  lining  of  a 
basic  Bessemer  converter  is  of  some  basic  material,  such 
as  dolomitic  limestone,  which  will  not  be  attacked  by  the 
basic  charge. 

The  basic  Bessemer  process  is  successful  only  with  pig 
iron  high  in  phosphorus  and  low  in  silicon.  Low  silicon  is 
necessary  in  order  that  the  charge  shall  not  be  acid  rather 
than  basic,  and  if  the  silicon  is  low,  there  will  not  be  enough 
fuel  (furnished -by  the  other  impurities  of  the  pig)  unless 
phosphorus  is  high.  There  are  practically,  no  American 
ores  which  produce  pig  iron  high  enough  in  phosphorus  for 
the  basic  Bessemer  process.1  American  pig  iron  too  high 
in  phosphorus  for  the  acid  Bessemer  process,  as  the  ordinary 
American  Bessemer  process  is  sometimes  called,  is  made 
into  steel  by  the  basic  open-hearth  process. 

General  Quality  and  Use  of  Bessemer  Steel. — The  Bes- 
semer process  of  steel  making  is  not  so  well  under  control 
as  is  the  open-hearth  process,  but  is  much  more  rapid,  and 

1  The  Basic  Bessemer  process  is  used  in  at  least  one  Canadian  steel  plant. 


MANUFACTURE  OF  STEEL  99 

owing  to  the  fuel  requirements  of  the  open-hearth  process 
the  Bessemer  process  is  cheaper.  However,  to  make  pig 
iron  suitable  for  the  acid  Bessemer  process,  a  special  grade 
of  ore,  low  in  phosphorus,  is  necessary  and  the  known  sup- 
plies of  this  ore  in  America  are  becoming  exhausted.  The 
general  opinion  of  metals  users  rates  acid  open-hearth  steel 
as  the  best  steel  produced  for  general  structural  purposes 
(special  steels  of  very  high  price  may,  of  course  be  of  higher 
quality,  but  their  use  in  general  structural  work  is  very 
limited),  basic  open-hearth  steel  is  usually  ranked  second 
to  acid  open-hearth  steel,  and  Bessemer  steel  is  ranked 
third.  Bessemer  steel  seems  to  be  inferior  to  open-hearth 
steel  in  reliability  and  uniformity.  There  is,  however,  no 
certain  way  to  distinguish  whether  a  given  lot  of  steel  was 
made  by  the  open-hearth  or  by  the  Bessemer  process  unless 
its  history  is  known. 

The  Bessemer  process  is  used  extensively  in  producing 
the  lower  grades  of  structural  steel  and  was  formerly  the 
principal  process  used  in  producing  railroad  rails.  The 
production  at  frequent  intervals  of  a  few  tons  of  steel, 
which  is  characteristic  of  the  Bessemer  process,  feeds  to 
the  rail-forming  rolls  a  steadier  supply  of  steel  than  is 
furnished  by  the  production  twice  a  day  of  a  large  quantity 
of  steel,  the  latter  manner  of  production  being  character- 
istic of  the  open-hearth  process.  By  the  use  of  a  battery  of 
open-hearth  furnaces,  the  tapping  of  different  furnaces  can 
be  timed  to  supply  steel  at  frequent  intervals,  and  owing 
to  the  increasing  scarcity  of  ores  of  iron  low  in  phosphorus 
the  Bessemer  process  is  less  used  in  rail-producing  steel 
plants,  than  the  basic  open-hearth  process. 

Duplex  Processes  of  Steel-making. — Combinations  of 
the  Bessemer  process  and  the  open-hearth  process  are  in 
use  at  some  steel  plants,  whereby  the  process  is  started  in 
a  Bessemer  converter,  and  the  final  purification,  including 
the  removal  of  phosphorus,  is  carried  out  in  a  basic  open- 
hearth  furnace.  This  duplex  process  has  become  one  of 
great  importance  in  the  leading  American  steel  plants. 

In  another  process  partial  refinement  of  pig  iron  is  ac- 


100  MATERIALS  OF  ENGINEERING 

complished  in  a  Bessemer  converter  or  an  open-hearth 
furnace,  or  by  a  combination  of  the  two,  and  final  refine- 
ment takes  place  in  an  electrically  heated  refining  furnace. 
The  advantages  and  limitations  of  the  electrically  heated 
steel  furnace  are  discussed  briefly  in  Chap.  IX. 

Selected  References  for  Further  Study 

STOUGHTON:  "The  Metallurgy  of  Iron  and  Steel,"  New  York,  1911,  Chaps. 
Ill  and  V.  An  excellent  general  treatise  by  an  American  metallurgist. 

CAMPBELL:  "The  Manufacture  and  Properties  of  Iron  and  Steel,"  New 
York,  1907,  Chaps.  VI,  VII.  An  excellent  general  treatise,  somewhat 
technical,  by  an  American  metallurgist. 

HARBORD  AND  HALL:  "The  Metallurgy  of  Steel,"  London,  1916,  Vol.  1, 
Chaps.  I-V,  inclusive.  A  comprehensive  treatise  by  two  British  met- 
allurgists. 

MACFARLANE:  "The  Principles  and  Practice  of  Iron  and  Steel  Manufac- 
ture," London,  1906,  Chaps.  VII-IX,  inclusive.  A  concise  text  by  a 
British  metallurgist. 


CHAPTER  IX 

CEMENTATION  STEEL,  CRUCIBLE  STEEL,  AND 
ELECTRIC-FURNACE  STEEL 

The  Cementation  Process. — The  oldest  process  of  steel- 
making  is  called  the  cementation  process.  This  process, 
still  in  use  in  England  for  making  fine  grades  of  cultery 
steel,  uses  as  raw  material  small  bars  of  wrought  iron  (see 
Chap.  VI).  These  are  packed  in  cast-iron  boxes  and 
each  bar  is  surrounded  with  powdered  charcoal.  The 
boxes,  sealed  tight,  are  then  heated  in  a  tall  conical  fur- 
nace up  to  a  temperature  of  1,200°F.,  and  this  heat  is 
continued  for  several  days.  At  this  temperature  carbon 
is  readily  soluble  in  iron  and  for  every  24  hr.  of  heating  the 
wrought  iron  is  carbonized  to  a  depth  of  about  %  in.  This 
carburizing  period  may  extend  over  a  week  or  more,  and 
is  followed  by  a  slow  cooling  period.  The  bars,  when  re- 
moved from  the  cast-iron  boxes,  are  covered  with  gas 
blisters,  and  the  steel  is  called  blister  steel.  These  bars  of 
blister  steel  are  worked  or  drawn  under  a  hammer,  piled 
together,  heated  to  a  welding  heat  and  redrawn  under  a 
hammer,  producing  shear  steel. 

Cementation  Steel,  Case -carbonized  Steel. — Cementa- 
tion steel  is  very  expensive,  but  of  a  very  high  quality. 
The  cementation  process  has  never  been  used  to  any  great 
extent  in  the  United  States  for  making  steel,  but  a  similar 
process,  the  case-carbonizing  or  case-hardening  process, 
is  in  common  use  for  giving  wrought  iron  or  soft  steel  a 
hard  "skin"  of  high-carbon  steel.  In  this  process  small 
articles  of  wrought  iron  or  soft  steel  are  packed  in  some 
carbonizing  agent,  such  as  charcoal,  leather  scraps,  bone 
dust,  or  potassium  ferrocyanide,  or  are  exposed  to  a  hy- 
drocarbon gas  and  heated  for  a  short  period  of  time.  The 
case-carbonizing  process  is  the  same  in  principle  as  the 

101 


102 


MATERIALS  OF  ENGINEERING 


cementation  process.  The  hardness  given  by  case  hard- 
ening is,  however,  only  a  surface  hardness,  and  the 
strength  of  a  case  hardened  part  is  increased  very  slightly 
by  the  process . 

The  Crucible  Process. — A  process  which  is  in  wide  use 
for  producing  high-grade  steel  is  the  crucible  process.  Like 
the  cementation  process,  it  depends  on  the  solubility  of 
carbon  in  hot  iron,  but  in  the  crucible  process  the  iron  is 


FIG.  37. — Diagram  of  crucible-steel  furnace. 

actually  melted,  and  the  carbonization  takes  place  much 
more  rapidly  than  in  making  cementation  steel. 

In  the  crucible  process,  the  charge  consists  of  powdered 
charcoal  and  bars  of  wrought  iron  and  other  ingredients 
containing  elements  desired  in  the  finished  steel,  such  as 
manganese,  tungsten,  etc.  The  charge  is  placed  in  clay 
or  graphite  crucibles.  The  crucibles  are  covered  to  keep 
out  air  and  placed  in  a  furnace  (Fig.  37) .  The  temperature 
is  so  high  that  the  wrought  iron  is  actually  melted,  and  in 
4  or  5  hr.  it  has  combined  with  the  carbon  and  with  other 
elements  present  in  the  substance  placed  in  the  crucible. 


CEMENTATION  AND  ELECTRIC-FURNACE  STEEL  103 

The  crucible  is  allowed  to  stand  quietly  in  the  furnace  for 
half  an  hour  after  the  carbonization  is  complete ;  this  allows 
any  gas  bubbles  in  the  molten  mass  to  escape,  and  after 
this  quiet  half  hour,  or  " killing"  as  it  is  called,  the  crucible 
is  lifted  out  of  the  furnace,  and  the  steel  poured  into  ingot 
molds.  The  ingots  formed  in  the  molds  are  afterward 
rolled  or  hammered  into  rods  and  bars  for  commercial 
purposes.  Fig.  37  gives  a  diagram  of  a  gas-fired  crucible 
furnace.  Fig.  38  gives  a  view  in  a  crucible-steel  plant. 


Courtesy  of  the  Columbia  Steel  Co. 
FIG.  38. — View  in  crucible-steel  plant.     Columbia  Steel  Co.,  Chicago  Heights,  111. 

In  the  United  States  crucible  furnaces  are  usually  re- 
generative furnaces  using  gas  as  fuel;  in  England  coke- 
fired  furnaces  are  in  use.  Each  crucible  contains  about 
75  Ib.  of  metal  and  is  handled  by  one  man.  The  labor 
of  handling  the  hot  crucible  is  severe.  Only  the  best 
quality  of  wrought  iron  or  very  high  grade  steel  scrap 
should  be  used.  The  heat  required  to  melt  the  wrought 
iron  is  very  great.  All  of  these  causes  combine  to  make 
the  cost  of  crucible  steel  five  to  twenty  times  that  of  open- 
hearth  steel  or  of  Bessemer  steel,  and  limits  its  use  to  small 
parts,  such  as  tools,  small  shafts  and  axles,  etc.,  which 


104  MATERIALS  OF  ENGINEERING 

must  be  very  hard  or  very  strong.  Crucible  steel  is,  how- 
ever, less  expensive  than  cementation  steel. 

The  superiority  of  cementation  steel  and  of  crucible  steel 
over  Bessemer  steel  and  open-hearth  steel  is  due  in  large 
measure  to  the  fact  that  crucible  steel  and  cementation 
steel  during  their  process  of  making  are  heated  in  closed 
vessels  out  of  contact  with  air.  If  in  the  open-hearth 
process  or  in  the  Bessemer  process  it  is  attempted  to  carry 
the  removal  of  impurities  so  far  as  to  give  steel  of  the  quality 
of  crucible  steel,  the  iron  itself  becomes  oxidized  from 
the  air  present.  To  produce  the  highest  grades  of  steel 
the  heating  must  be  done  out  of  contact  with  air. 

The  Electric  Furnace  for  Refining  Steel. — If  the  heat 
necessary  to  refine  steel  is  produced  by  an  electric  current, 
the  heat  can  be  produced  in  direct  contact  with  the  charge 
of  a  steel  furnace,  but  without  the  presence  of  air.  Direct 
contact  with  the  charge  of  a  furnace  means  a  more  econom- 
ical utilization  of  heat  than  is  possible  where  the  heat  must 
be  supplied  through  the  walls  of  a  crucible,  and  freedom 
from  contact  with  air  makes  possible  the  highest  refinement 
of  steel  without  danger  of  oxidation.  For  producing  low 
temperatures,  electrically  produced  heat  is  very  expensive 
as  compared  with  heat  produced  by  burning  gas  or  coke; 
however,  for  high  temperatures  the  relative  cost  of  elec- 
trically produced  heat  is  less.  With  fuel-produced  heat 
more  air  must  be  used  to  produce  high  temperatures,  and 
this  air  must  be  heated  as  well  as  the  charge  of  the  furnace. 
With  fuel-produced  heat  the  higher  the  temperature  the 
greater  the  waste  of  heat  in-  heating  the  air  required  to 
produce  combustion  and  the  higher  the  relative  cost  of  the 
heat.  With  electrically  produced  heat  the  cost  is  nearly 
proportional  to  the  temperature.  For  some  limiting  tem- 
perature, then  ,  the  cost  of  electrically  produced  heat  will 
be  equal  to  that  of  fuel-produced  heat,  while  for  still  higher 
temperatures  the  cost  of  electrically  produced  heat  will  be 
actually  less  than  that  of  fuel-produced  heat.  The  tem- 
peratures used  in  refining  steel  approach  this  limiting  tem- 
perature under  the  conditions  prevalent  at  most  steel 


CEMENTATION  AND  ELECTRIC-FURNACE  STEEL  105 

plants,  and  it  becomes  feasible  to  use  the  electric  furnace 
for  the  final  refining  processes  of  steel-making. 

Duplex  and  Triplex  Processes  of  Steel  Making,  using 
the  Electric  Furnace. — The  non-oxidizing  nature  of  elec- 
trically produced  heat,  and  the  relatively  diminished  cost 
of  such  heat  at  high  temperatures  are  utilized  in  composite 
processes  of  steel  making  which  use  the  open-hearth  furnace, 
the  Bessemer  converter,  or  both  for  preliminary  refining  of 
pig  iron,  and  the  electric  furnace  for  the  final  refining. 
In  the  triplex  process  pig  iron  from  blast  furnaces  is  first 
fed  to  "mixers"  (see  p.  95)  and  then  to  Bessemer  con- 
verters where  silicon,  manganese  and  carbon  are  nearly 
all  removed;  the  steel,  still  molten,  is  then  transferred  to 
basic  open-hearth  furnaces,  where  phosphorus  is  removed 
by  the  basic  oxidizing  slag,  and  the  steel  is  recarburized; 
the  steel  is  then  removed  to  an  electric  furnace  where  it  is 
deoxidized  and  desulphurized  by  the  action  of  the  slag 
11  blanket"  in  the  electric  furnace.  Usually  only  a  part 
of  the  product  of  the  basic  open-hearth  furnace  is  thus 
treated  in  the  electric  furnace,  the  larger  part  is  poured 
into  ingot  molds  after  the  open-hearth  furnace  treatment 
and  is  thus  duplex-process  steel.  The  composite  processes 
of  steel  making,  involving  final  refining  in  the  electric 
furnace,  give  promise  of  great  importance  in  the  steel 
industry. 

Types  of  Electric  Steel  Furnaces. — Electric  furnaces  for 
refining  of  steel  are,  in  general,  of  two  types:  the  arc  fur- 
nace, and  the  induction  furnace.  In  the  arc  furnace  the 
heat  is  produced  by  means  of  an  electric  arc  between  carbon 
electrodes  or  between  a  carbon  electrode  and  the  furnace 
charge.  Fig.  39  shows  a  diagram  and  Fig.  40  a  general 
view  of  a  typical  arc -type  electric  furnace.  The  electrodes 
project  through  the  roof  of  the  furnace.  The  charge  rests 
on  the  hearth  and  is  purified  by  the  action  of  a  slag  formed 
on  top  of  it  which  contains  decarbonizing,  dephosphorizing, 
and  desulphurizing  ingredients,  as  may  be  needed. 

Fig.  41  shows  in  diagram  an  induction  type  of  electric 
furnace.  The  charge  cc  when  melted  forms  the  short- 


106 


MATERIALS  OF  ENGINEERING 


CONDUCTORS 


FIG.  39. — Diagram  of  arc-type  electric  steel  furnace. 


Courtesy  of  Illinois  Steel  Co. 

FIG.  40. — Arc- type    electric   steel  furnace,  Illinois  Steel  Co.,  So.   Chicago,  111. 
The  furnace  shown  has  three  electrodes  and  uses  three-phase  current. 


CEMENTATION  AND  ELECTRIC-FURNACE  STEEL  107 

circuited  secondary  winding  of  a  transformer  of  which  the 
primary  winding  is  shown  at  a.  To  start  such  a  furnace 
it  is  necessary  that  a  ring  of  metal  be  placed  in  the  grooves 
c  or  that  the  charge  be  supplied  molten.  In  general,  in- 
duction furnaces  are  used  to  produce  small  quantities  of 
steel. 

The  electric  process  of  steel-refining  is  very  recent,  but 
is  claimed  to  produce  steel  of  as  high  grade  as  the  crucible 
process.  Rail  steel  refined  by  the  electric  process  is  in  use 
to  a  limited  extent  and  seems  to  be  of  excellent  quality. 
The  electric  refining  process  is  of  special  promise  for  pro- 
ducing steel  free  from  phosphorus,  sulphur,  oxide,  and  gas 
bubbles. 


FIG.  41. — Diagram  showing  principle  of  induction  electric  furnace   for  refining 

steel. 

Electric  Reducing  of  Iron  Ore. — In  certain  localities, 
notably  in  Sweden  and  Norway,  where  abundant  water 
power  makes  electricity  inexpensive,  iron  ore  is  reduced 
directly  in  electrically  heated  furnaces.  This  produces  a 
very  pure  grade  of  iron,  but,  except  in  locations  where 
electric  power  can  be  produced  very  cheaply,  the  direct 
reduction  of  ore  in  electrically  heated  furnaces  is  far  too 
expensive  to  be  practicable. 

Selected  References  for  Further  Study 

STOUGHTON:  "The  Metallurgy  of  Iron  and  Steel,"  New  York,  1911,  Chaps. 
Ill,  IV,  XV,  XVII.  An  excellent  general  treatise  by  an  American 
metallurgist. 


108  MATERIALS  OF  ENGINEERING 

HARBORD  AND  HALL:  "The  Metallurgy  of  Steel,"  London,  1916,  Vol.  1, 
Chaps.  X,  XI,  XIII.  A  comprehensive  treatise  by  two  British  metal- 
lurgists. 

LYON  AND  KEENEY:  Electric  Furnaces  for  Making  Iron  and  Steel,  U.  S. 
Bureau  of  Mines,  Bulletin  37. 

NEILSON:  The  Manufacture  of  Crucible  Steel,  Iron  Age,  July  2,  1914. 

HAMMOND:  The  Manufacture  of  Tool  Steel,  Iron  Age,  Oct.  3,  1912. 

SYKES:  The  Status  of  the  Electric  Steel  Furnace,  Iron  Age,  Oct.  16,  1913. 

SUVERKROP:  Manufacture  of  Electric  Tool  Steel,  American  Machinist,  Feb. 
28,  1918. 


CHAPTER  X 
IRON  AND  STEEL  CASTINGS 

Cast  Iron;  the  Cupola. — When  pig  iron  is  to  be  used  for 
the  direct  production  of  parts  of  structures  or  machines,  it 
is  remelted  and  cast  into  molds.  This  re  melting  is  neces- 
sary because  the  product  of  any  one  blast  furnace  is  usually 
quite  variable  in  quality,  and  because  a  mixture  of  differ- 
ent varieties  of  pig  iron  usually  produces  a  better  grade  of 
castings  than  does  any  one 
grade  of  pig  iron;  also  because 
by  different  mixtures  different 
qualities  of  casting  can  be  pro- 
duced for  special  purposes. 
Pig  iron  remelted  and  cast  into 
molds  is  called  cast  iron.  The 
remelting  is  usually  done  in  a 
cupola  which  is  somewhat  like 
a  small  blast  furnace.  Fig.  42 
shows  a  diagram  of  a  cupola. 
A  blast  of  air  under  light  Tuyeres. 
pressure  is  blown  through 
tuyeres  at  the  bottom  of  the 
stack,  which  is  charged  from 
the  top  with  alternate  layers 
of  coke  (fuel)  and  pig  iron 
mixed  with  scrap.  The  pig 
and  the  scrap  melt  and  trickle 
down  through  the  fuel  to  the 
bottom  of  the  cupola,  and  a 

slag  forms  which  floats  on  top  of  the  melted  iron.  As  in  a 
blast  furnace  the  slag  is  drawn  off  through  a  slag  hole  and 
the  iron  through  a  tap  hole.  The  principal  differences 
between  the  action  of  a  blast  furnace  and  that  of  a  cupola 

109 


From  Blower 


Diagram  of  foundry  cupola 
for  cast  iron. 


110  MATERIALS  OF  ENGINEERING 

are  that  in  the  cupola  there  is  no  marked  chemical  change 
in  the  raw  material  as  the  process  progresses,  and  that 
the  proportion  of  fuel  to  iron  in  a  cupola  is  only  about 
20  per  cent. 

Castings  of  cast  iron  are  made  in  molds  of  loam  or  of 
sand.  Ordinary  cast  iron  contains  from  2.5  to  6  per  cent, 
of  carbon,  which  may  be  present  as  combined  carbon, 
graphite  flakes,  or  finely  divided  graphite. 

Air-furnace  Iron. — A  limited  amount  of  cast  iron  of 
superior  quality  is  produced  by  remelting  in  the  air  furnace 
rather  than  in  the  cupola.  An  air  furnace  is  not  dissimilar 
in  general  appearance  to  a  puddling  furnace.  The  iron  is 
melted  on  a  hearth,  while  the  fuel,  usually  soft  coal,  is 
burned  on  a  grate.  Iron  melted  in  an  air  furnace  is  usually 
superior  to  cupola  iron  because  it  is  less  contaminated  by 
the  impurities  (especially  sulphur)  in  the  fuel.  The  melt- 
ing is  under  better  control  in  an  air  furnace  than  in  a  cupola, 
and  in  an  air  furnace  some  purification  of  the  pig  iron  usu- 
ally takes  place.  The  intimate  mixture  of  the  charge  and 
the  fuel  in  the  cupola,  while  it  tends  to  contamination  of 
the  charge,  does,  however,  cause  high  economy  of  fuel. 
Air-furnace  iron  requires  about  twice  as  much  fuel  as 
cupola  iron  and  hence  is  more  expensive  than  is  cupola 
iron. 

Open-hearth  Furnaces  for  Cast  Iron. — Open-hearth  fur- 
naces such  as  are  used  in  steel-making  are  used  to  a  limited 
extent  for  producing  cast  iron.  The  advantages  of  their 
use  are  greater  economy  of  fuel  than  the  air  furnace,  and 
less  contamination  of  charge  by  fuel  than  in  the  cupola. 
The  open-hearth  to  be  at  all  economical  must  be  operated 
continuously  day  and  night,  and  this  can  be  done  only  in 
foundries  with  a  very  large  floor  space  for  setting  up  molds, 
as  the  preparation  of  molds  cannot  be  well  done  by  artificial 
light. 

Semi-Steel. — Semi-steel  is  the  rather  misleading  name 
given  to  a  product  of  the  air  furnace  or  of  the  cupola  which 
is  made  by  melting  a  mixture  of  20  to  50  per  cent,  of  steel 
scrap  with  pig  iron.  The  product  is  a  cast  iron  of  high 


IRON  AND  STEEL  CASTINGS 


111 


strength  and  low  carbon  content ;  it  is  not  steel.  Semi-steel 
is  used  to  make  iron  castings  in  which  strength  is  important. 
Punch  and  shear  frames,  parts  of  hydraulic  presses  and 
other  machines  subjected  to  severe  stresses  are  frequently 
made  of  semi-steel. 


a.   White  cast  iron. 
J'holomicrograph  by  F.  E.  Rowland.     Magnification  100  times. 


•     6.  Gray  cast  iron.  c.  Malleable  cast  iron. 

Photomicrograph  by  H.  T.  Manuel.  Photomicroaraph  by  F.  E.  Rowland. 

Magnification  45  times.  Magnification  100  times. 

FIG.  43. — Crystalline  structure  of  cast  iron. 

Gray  Cast  Iron,  White  Cast  Iron,  Chilled  Cast  Iron.— If 

cast  iron  is  allowed  to  cool  slowly,  a  large  part  of  the  carbon 
in  the  cast  iron  will  be  found  in  the  form  of  crystals  of 
graphite  (see  Fig.  436).  Such  cast  iron  is  known  as  gray 


112  MATERIALS  OF  ENGINEERING 

cast  iron.  The  crystals  of  graphite  make  gray  cast  iron  very 
brittle  as  compared  with  steel  or  wrought  iron.  Gray  cast 
iron  is  soft  and  easily  machined.  Its  tensile  strength  is 
about  one-third  that  of  soft  steel,  and  its  compressive 
strength  fully  equal  to  that  of  soft  or  medium  steel.  Ordi- 
nary cast  iron  for  machine  frames,  stoves,  etc.,  is  gray 
cast  iron. 

If  cast  iron  is  very  rapidly  cooled  in  the  mold,  the  carbon 
will  not  have  time  to  be  precipitated  as  graphite,  but  will 
remain  chemically  combined  with  the  iron,  giving  what  is 
known  as  white  cast  iron  (see  Fig.  43a).  White  cast  iron 
is  stronger  than  gray,  but  is  much  harder,  so  hard  that  it 
can  not  be  machined,  and  is  extremely  brittle. 

If  a  part  of  a  casting  is  suddenly  cooled  while  other  parts 
are  allowed  to  cool  slowly,  the  resulting  casting  will  be 
white  cast  iron  where  sudden  cooling  occurs,  and  gray  cast 
iron  where  slow  cooling  occurs.  Local  sudden  cooling 
(chill)  can  be  produced  by  lining  a  part  of  the  mold  with 
metal,  which  rapidly  conducts  the  heat  away  from  that  part 
of  the  casting.  The  rims  of  cast-iron  wheels  are  chilled 
to  resist  wear  while  the  center  is  of  gray  iron,  which  can  be 
bored  for  the  axle,  and  which  resists  shock  better  than  does 
the  very  brittle  chilled  iron. 

Malleable  Cast  Iron. — Malleable  cast  iron  is  produced 
from  white  cast  iron  by  an  annealing  process.  The  white 
cast  iron  is  cast  directly  into  the  desired  shapes,  and  the 
annealing  process  does  not  materially  change  these  shapes. 
The  castings  of  white  iron  are  packed  in  pulverized  iron 
oxide  (sometimes  lime  or  even  sand  is  used)  and  are  heated 
to  a  temperature  of  about  1,300°F.  for  several  days.  This 
annealing  process  (a  reverse  of  the  cementation  process  for 
making  steel)  changes  the  combined  carbon  of  the  'white 
cast  iron  to  graphite,  which,  however,  occurs  in  finely 
divided  particles  called  temper  carbon  rather  than  in  flakes 
(see  Fig.  43c).  Malleable  cast  iron  has  rather  more  than 
twice  the  tensile  strength  of  gray  cast  iron,  and  has  very 
much  higher  ductility  than  gray  cast  iron;  the  ductility 
of  malleable  cast  iron  is  about  a  quarter  of  the  ductility  of 


IRON  AND  STEEL  CASTINGS 


113 


structural  steel.  Malleable  cast  iron  is  used  for  small 
castings  for  which  forged  steel  is  too  expensive,  and  in 
which  the  material  should  have  considerable  ductility; 
for  example,  hubs  of  wagon  wheels,  small  fittings  for  rail- 
way rolling  stock,  parts  for  agricultural  machinery,  pipe 
fittings,  door  hinges,  etc. 

Malleable  cast  iron  is  not  so  strong  as  cast  steel,  but  can 
be  poured  at  a  lower  temperature,  and  gives  castings 
truer  to  pattern  than  are  steel  castings. 

Steel  Castings. — In  recent  years  there  has  been  a  great 
development  in  the  production  of  steel  parts  for  machines 
and  structures  by  direct  casting.  This  has  been  especially 


Gafe- 


.— Vents—. 


Riser  or  5i/ik  Head 


FIG.  44. — Mold  for  steel  casting. 

noticeable  in  the  production  of  railway  equipment,  such  as 
locomotive  frames,  car  couplers,  draft  rigging,  etc.  Steel 
castings  may  be  made  either  of  open-hearth  steel  or  of 
Bessemer  steel;  but  open-hearth  steel  castings  are  much 
more  common.  The  molten  steel  from  the  furnace  or 
converter  is  poured  directly  into  molds.  Steel  castings 
may  be  made  of  almost  any  desired  carbon  content.  There 
are  a  large  number  of  complex  foundry  problems  involved 
in  producing  steel  castings.  To  insure  soundness  of  steel 
in  castings  and  freedom  from  cavities  formed  during  cooling 
it  is  usually  necessary  to  pour  the  castings  with  large  masses 
of  steel,  called  sink  heads,  so  placed  in  the  mold  that  molten 


114  MATERIALS  OF  ENGINEERING 

steel  may  flow  from  them  to  any  part  of  the  casting  where 
there  is  a  tendency  toward  the  formation  of  cavities  due 
to  quick  cooling  (see  Fig.  44). 

The  material  of  steel  castings  is  not  so  strong  nor  so 
tough  as  is  forged  steel  of  the  same  chemical  composition. 
As  they  come  from  molds  steel  castings  usually  have 
severe  internal  stresses  set  up  in  them  by  uneven  cooling. 
These  internal  stresses  may  be  greatly  relieved,  and  the 
quality  of  the  material  in  various  parts  of  the  casting  made 
more  nearly  uniform,  by  annealing  the  finished  casting, 
and  this  is  very  frequently  done. 

For  many  purposes  today  steel  castings  are  becoming 
available  in  place  of  steel  forgings  on  account  of  the  greater 
ease  of  making  castings  of  complicated  shape,  and  steel 
castings  are  also  displacing  gray  iron  castings  for  many 
machine  parts  in  which  strength  and  toughness  are  prime 
requisites. 

Values  for  the  strength  of  gray  cast  iron,  malleable 
cast  iron,  and  steel  castings  are  given  in  the  table  of 
strength  and  ductility  of  ferrous  metals  (Table  5,  p.  150). 

Selected  References  for  Further  Study 

STOUGHTON:  "The  Metallurgy  of  Iron  and  Steel,"  New  York,  1911,  Chaps. 

IX,  XIII. 
WEST:  "Metallurgy  of  Cast  Iron,"  Cleveland,  Ohio,  1907.     An  excellent 

general  reference  book  by  an  American  foundryman. 
HARBORD  AND  HALL:  "The  Metallurgy  of  Steel,"  London,  1916,  Vol.   1. 

Chap.  IX. 
MILLS:  "The  Materials  of  Construction,"  New  York,  1915,  Chaps.  XII, 

XIII. 

HALL:  "The  Steel  Foundry,"  New  York,  1917. 
MOLDENKE:  "The  Principles  of  Iron  Sounding,"  New  York,  1917. 
THE   AMERICAN   MALLEABLE   CASTINGS   ASSOCIATION:  "Malleable   Iron," 

Cleveland,  Ohio,  1918.     This  is  a  trade  pamphlet  which  gives  a  concise 

statement  of  the  manufacture  and  properties  of  malleable  cast  iron. 


CHAPTER  XI 

THE  MECHANICAL  TREATMENT  OF  STEEL 
ROLLING,  FORGING  AND  PRESSING 

Uses  of  Rolled  Steel. — Among  the  materials  used  by 
engineers  for  structures  and  machines,  rolled  steel  occupies 
a  place  of  importance  second  to  none.  Bolts,  nuts,  rivets, 
screws,  nails,  structural  shapes  (such  as  /-beams,  channels, 
and  angles),  steel  plates  for  metal  stacks,  tanks,  and  boiler 
shells,  shafts  and  axles,  railway  rails,  wire,  chain,  thin  sheet 
metal  for  roofing,  siding  and  tubing,  gas  and  water  pipe — 
these  are  some  common  articles  made  from  rolled  steel. 

Steel  Ingots. — When  steel  which  comes  from  the  fur- 
nace or  the  converter  is  to  be  rolled  or  forged,  it  is  poured 


Section    at 
A-  B 


FIG.  45. — Five-ton  ingot  mold. 


into  large  ingot  molds  mounted  on  cars.  Each  mold  con- 
tains from  3  to  10  tons  of  steel  (see  Fig.  45).  These  ingot 
molds  after  filling  are  transferred  on  ingot  cars  to  a  yard 
under  a  crane  which  strips  the  ingot  molds  from  the  steel 
after  solidification  has  taken  place.  The  ingot,  solid  but 

115 


116  MATERIALS  OF  ENGINEERING 

still  hot  is  then  transferred  to  a  "soaking"  furnace  where 
it  is  kept  at  red  heat  until  it  can  be  carried  to  the  rolls  or 
to  the  steam  hammer  for  final  shaping. 

Defects  in  Steel  Ingots. — As  the  steel  solidifies  in  the 
ingot  mold  there  are  three  actions  which  tend  to  make  the 
ingot  vary  in  quality  in  various  parts:  (1)  the  steel  in 
contact  with  the  surface  of  the  mold  solidifies  first,  and, 
contracting,  tends  to  draw  the  remain- 
ing steel  away  from  the  center  of  the 
mold,  the  result  being  a  cavity  or 
"pipe"  in  the  upper  part  of  the  ingot 
(see  Fig.  46) ;  (2)  the  carbon,  the  sulphur, 
and  the  phosphorus  instead  of  remain- 
ing uniformly  distributed  throughout 
the  ingot  tend  to  gather  together  or 
"segregate"  in  spots,  especially  in  the 
upper  part  of  the  ingot;  (3)  gases 
escaping  as  the  steel  is  on  the  point  of 
solidification  may  leave  minute  blow- 
holes "honey-combing"  the  upper  part 
of  the  ingot.  (See  h,  Fig.  46.) 

Effects  of  "Pipes"  and  Their  Pre- 
vention.— If  that  part  of  the  ingot  in 


rolls  or  under  a  forging  hammer  at  a 
temperature  as  high  as  the  welding  temperature  of  steel  the 
walls  of  the  cavity  will  be  welded  together;  however,  this 
welding  action  is  not  at  all  sure  under  usual  rolling  or 
forging  conditions,  and  a  pipe  in  the  ingot,  if  rolled  into 
rails,  plates,  or  shapes  is  liable  to  produce  a  longitudinal 
seam  in  the  finished  product.  This  is  especially  dangerous 
in  rails. 

As  piping  occurs  in  the  upper  portion  of  the  ingot,  its 
evil  effect  may  be  prevented  by  cutting  off  or  "cropping" 
the  top  of  the  ingot.  This  cropped  portion  is  then  used  as 
scrap  for  the  open-hearth  furnace.  It  is  usually  necessary 
to  crop  from  20  to  30  per  cent,  of  the  ingot  in  order  to  insure 
freedom  from  piping  in  the  finished  steel.  The  reheating 


STEEL  ROLLING,  FORGING  AND  PRESSING      117 

of  the  cropped  portion  of  the  ingot  is  expensive,  and  many 
attempts  have  been  made  to  prevent  the  formation  of 
pipes,  or  to  lesses  their  extent  and  consequently  to  lessen 
the  amount  of  cropping  of  ingot  required.  One  method  of 
minimizing  piping  which  is  in  successful  use  especially  in 
British  steel  works  consists  in  compressing  the  ingot  in  a 
powerful  hydraulic  press  while  the  interior  is  still  in  a 
pasty  state.  Another  method  recently  proposed  consists 
in  heating  the  top  of  the  ingot  in  the  ingot  mold  so  that  its 
solidification  shall  be  retarded  and  the  solidification  of  the 
whole  ingot  made  more  nearly  uniform.  Slow  pouring  of 
the  steel  into  the  ingot  mold  lessens  piping  by  allowing  the 
metal  of  the  mold  to  become  heated  gradually,  and  the 
temperature  changes  in  the  ingot  to  take  place  more  gradu- 
ally. 

Effects  of  Segregation  and  its  Prevention. — The  evil 
effects  of  segregation  are  due  to  the  variation  of  quality  in 
steel  which  it  causes  in  various  parts  of  the  finished  piece. 
The  segregation  of  carbon  causes  some  parts  of  the  rolled 
product  to  be  too  hard,  and  some  to  be  too  weak.  If, 
for  example,  the  carbon  of  a  steel  rail  is  segregated  so  that 
there  is  higher  carbon  content  in  the  base  than  in  the  head, 
there  results  a  rail  with  a  head  so  soft  that  it  soon  wears 
out,  and  with  a  base  so  hard  that  it  is  unduly  brittle.  The 
segregation  of  phosphorus  may  cause  cold  shortness  in 
spots,  that  is,  there  will  be  very  brittle  spots  in  a  piece  of 
steel  in  which,  as  a  whole,  the  phosphorus  is  not  danger- 
ously high.  The  segregation  of  sulphur  may  cause  trouble 
in  rolling  the  steel  on  account  of  red  shortness,  that  is, 
brittleness  when  hot.  Segreation  is  minimized  by  quick 
cooling  of  -the  ingot.  The  contradictory  requirements  of 
slow  colling  to  minimize  piping,  and  quick  cooling  to  mini- 
mize segregation  render  the  handling  of  any  lot  of  steel  a 
matter  of  earful  study  in  order  that  the  best  quality  may 
be  secured. 

Effects  of  Honeycombing  and  Its  Prevention. — The  min- 
ute blow-holes  in  " honeycombed"  steel  render  its  strength 
uncertain,  especially  under  repeated  stress,  which  causes 


118  MATERIALS  OF  ENGINEERING 

cracks  to  spread  from  the  minute  blow-holes.  The  steel 
seems  "rotten."  Honeycombing  is  minimized  by  allow- 
ing the  steel  to  remain  quietly  in  the  ladle  before  pouring 
into  the  ingot  mold  until  the  entrained  gases  have  had  an 
opportunity  to  escape.  This  process  is  known  as  "  killing  " 
the  steel,  and  steel  from  which  gases  are  freely  bubbling  is 
known  as  "wild"  steel.  A  small  quantity  of  aluminium 
thrown  into  the  ladle  hastens  the  "killing"  process.  The 
use  of  titanium  is  said  to  produce  the  same  result.  The 
superior  quality  of  electric-furnace  steel  is  due,  in  part, 
to  the  freedom  from  contact  of  boiling  steel  with  air  and 
gas  during  the  process  of  manufacture,  with  the  conse- 
quent absorption  of  gas  by  the  steel. 

The  Rolling  Mill.  —  In  cooling  the  temperature  of  the 
surface  of  the  ingot  is  lowered  below  that  required  for  sue- 


FIG.  47.  —  Different  cross-sections  during  the  process  of  rolling  a  steel  rail. 

cessful  rolling,  and  the  ingot  is  reheated  in  a  "soaking  pit,  " 
usually  fired  by  waste  gas  from  a  blast  furnace  or  an  open- 
hearth  furnace.  From  the  soaking  pit  the  ingot  is  passed 
through  a  series  of  ten  or  a  dozen  rolls  which  reduce  its 
cross-section  and  increase  its  length  until  the  desired  shape 
is  "reached.  Fig.  47  shows  the  shapes  given  the  steel  by 


FIG.  48.  —  Common  rolled  sections. 

successive  rolls  as  an  ingot  is  rolled  into  a  railroad  rail. 
For  producing  plates  rods,  rails,  structural  shapes,  etc., 
the  steel  ingots  are  pased  through  rolls  which  gradually 
reduce  the  cross-section  of  the  ingot  to  that  of  the  desired 
product.  Fig.  48  shows  some  common  rolled  sections. 


STEEL  ROLLING,  FORGING  AND  PRESSING      119 

The  rolls  to  which  the  ingot  is  first  sent  are  known  as 
"cogging"  rolls,  or  as  a  " blooming  mill."  The  rolls  are 
usually  two  in  number  for  such  a  mill,  which  is  known  as 
a  " two-high"  mill,  and  after  the  ingot  is  passed  through 
the  rolls  they  are  brought  closer  together,  the  direction  of 
rotation  reversed,  and  the  ingot  passed  back  through  them. 
Later  as  the  section  of  the  metal  approaches  its  final  form 
" three-high"  rolls  are  used.  Figs.  49  and  50  show  a  two- 
high,  and  Fig.  51  shows  a  three  high  rolling  mill.  It  will 
be  evident  that  it  is  not  necessary  to  reverse  the  three-high 
mill  in  order  to  send  the  ingot 
back  through  it.  Figs.  50  and  51 
also  show  the  rollers  and  the  table 
which  feed  the  steel  to  the  rolls; 
in  a  three-high  mill  this  table  can 

be  raised  and  lowered  so  that  the       FlG  49._Two.high  rolls. 
steel  may  be  fed  either  between 

the  upper  and  the  middle  or  between  the  middle  and 
the  lower  rolls.  The  actual  mechanism  for  raising  and 
lowering  the  table  involves  a  somewhat  complicated  lever 
system.  Fig.  52  shows  the  principle  of  a  " universal" 
rolling  mill.  The  distinguishing  feature  of  this  mill  is  the 
use  of  a  pair  of  vertical  rolls  to  finish  the  edge  as  well  as 
the  surface  of  the  steel  passed  through  it. 

Cold-rolled  and  Cold-drawn  Steel. — Rolling  steel  if 
done  at  a  proper  temperature  improves  its  quality.  Small 
cavities  are  closed  up,  and  all  the  particles  are  packed  close- 
ly together.  Ordinary  steel  is  rolled  at  a  red  heat,  and  the 
strength  and  the  ductility  of  the  steel  are  raised  by  rolling. 
If  steel  is  brought  to  its  final  size  by  rolling  at  temperatures 
below  red  heat,  it  is  known  as  cold-rolled  steel.  The  cold- 
rolling  of  steel  increases  its  static  strength  above  that  of 
hot-rolled  steel,  but  diminishes  it  ductility.  The  same 
effect  may  be  produced  in  rods  and  wire  by  drawing  the 
steel  through  hardened  steel  dies.  Cold-drawn  and  cold- 
rolled  steel  are  used- extensively  for  shafting.  The  process 
leaves  the  steel  with  a  very  smooth  surface,  and  the  cold- 
drawn  or  cold-rolled  steel  may  be  produced  so  true  to  shape 


120 


MATERIALS  OF  ENGINEERING 


STEEL  ROLLING,  FORGING  AND  PRESSING      121 

and  size  that  it  is  not  necessary  to  machine  it  before  using 
it  as  shafting.  One  drawback  to  the  use  of  cold-rolled 
or  cold-drawn  shafting  is  its  liability  to  "kink"  if  key  ways 
or  holes  are  cut  in  the  steel.  The  cold-rolling  process  sets 
up  very  high  stresses  in  the  metal,  and  in  cutting  a  keyway 


(^ 


:^£^©r4Jf$! 


FIG.  51. — Diagram  showing  principle  of  three-high  rolling  mill. 

or  hole  some  of  the  internal  stress  is  relieved,  and  to  pre- 
serve internal  equilibrium  of  stress  a  redistribution  of  the 
remaining  internal  stress  takes  place,  with  a  consequent 
distortion  of  the  shaft.  Test  results  seem  to  show  that 
cold-rolled  or  cold-drawn  steel  develops  no  greater  resist- 
ance to  fatigue  under  repeated  stresses  than  does  hot- 
rolled  steel  with  the  same  chemical  composition. 

Cold-drawing  steel  through 
hardened  steel  dies  produces  much 
the  same  effect  as  does  cold- 
rolling.  Cold-rolling  or  cold- 
drawing  distorts  the  crystalline 
structure  of  steel,  drawing  out  the 
crystals  in  the  direction  of  rolling. 
The  effects  of  rolling  may  be 
largely  removed  by  annealing  at 
proper  temperatures.  Fig.  53  gives  photomicrographs 
showing  the  effect  of  cold-working  on  the  crystalline  struc- 
ture of  steel.  Cold-drawn  or  cold-rolled  shafts  and  rods 


FIG.  52. — Diagram  showing 
principle  of  universal  rolling 
mill. 


122 


MATERIALS  OF  ENGINEERING 


may  be  obtained  up  to  3  in.  in  diameter.     Nearly  all  steel 
wire  is  cold-drawn. 


6.  15  per  cent,  reduction  from  original 
cross-section. 


Courtesy  of  Iron  Age. 

c.  60  per  cent,  reduction  from  original          d.  Annealed  after  cold-drawing, 
cross-section. 

FIG.  53. — Effect  of  cold-drawing  and  annealing  on  steel  wire  with  0.08  per 
cent,  carbon.  Photomicrographs  by  John  F.  Tinsley.  Magnification  80  times. 

Forging  and  Pressing  Processes. — Large  steel  objects 
such  as  heavy  shafts,  cannon,  thick  plate,  etc.,  are  shaped 
by  forging  hot  under  a  hydraulic  press  or  a  steam  hammer. 
In  general,  pressing  and  hammering  both  inprove  the 
quality  of  the  steel,  and  pressing  affects  the  steel  to  a 
greater  depth  than  does  hammering.  Hammering  because 
it  acts  by  means  of  a  large  number  of  blows  applied  to  all 


STEEL  ROLLING,  FORGING  AND  PRESSING      123 

parts  of  the  surface  of  the  object,  gives  greater  uniformity 
throughout  all  parts  of  thin  forgings  than  does  pressing. 
Small  objects  made  of  high-grade  crucible  steel,  such  as 
high-carbon  steel  rods,  stock  for  cutlery,  tools,  etc.,  are 
usually  hammered  to  shape.  If  a  large  number  of  objects 
of  the  same  size  is  to  be  produced,  and  if  a  better  quality 
of  material  is  desired  than  is  obtainable  in  steel  castings, 
these  articles  are  frequently  shaped  by  hammering  steel 
hot  between  hardened  steel  dies.  TJiis  process  is  called 
drop  forging. 


BuH"  welded      Lap -welded 

Pipe  Pipe 

o  <  >   o 


FIG.  54. — Diagram  of  pipe-welding  rolls. 

Steel  plate  is  pressed  into  shape  for  tanks,  boxes,  hollow 
cylinders,  boiler  heads,  etc.,  in  " flanging"  presses.  In 
these  presses  the  steel  is  shaped  by  means  of  steel  dies. 
Seamless  steel  tubing  is  formed  by  rolling  hot  steel  over  a 
long  mandrel.  Ordinary  steel  and  iron  pipe  is  made  from 
plates,  known  as  "skelp"  which  are  rolled  into  tube  shape, 
after  which  the  "skelp"  is  heated  to  welding  temperature 
and  passed  through  rolls  and  over  a  mandrel  as  shown  in 
Fig.  54. 

Selected  References  for  Further  Study 

STOUGHTON:  "The  Metallurgy  of  Iron  and  Steel,"  New  York,  1911,  Chaps. 

VII,  VIII. 

HARBORD  AND  HALL:  "The  Metallurgy  of  Steel,"  London,  1916,  Vol.  II. 
MATHIAS:  Progress  in  Steel  Mill  Roll  Design,  Iron  Age,  Oct.  30,  and  Nov.  6 

1913. 


124  MATERIALS  OF  ENGINEERING 

SPRINGER:  Seamlets  Steel  Tube  Manufacture,  Iron  Age,  Sept.  15,  1910. 

A  Symposium  on  Sound  Steel  Ingots,  Iron  Age,  Feb.  27,  1913. 

HOWE:  Control  of  Piping  and  Segregation  in  Ingots,  Iron  Age,  Oct.  28,  1915. 

SUVERKOP:  Manufacture  of  Cold-drawn  Shafting,  American  Machinist, 
July  16,  1914. 

CHARPY:  Hot  Deformation  and  the  Quality  of  Steel.  British  Iron  and 
Steel  Institute,  Fall  meeting,  1918.  This  paper  and  a  summary  of 
the  discussion  on  it  are  given  in  the  Iron  Age,  April  24  and  May  8,  1919. 


CHAPTER  XII 

THE  CRYSTALLINE  STRUCTURE  OF  IRON  AND 
STEEL  AND  ITS  SIGNIFICANCE;  THE  HEAT- 
TREATMENT  OF  STEEL;  WELDING 

The  Importance  of  the  Crystalline  Structure  of  Metals. 
The  strength  and  toughness  of  metals  depend  not  only 
on  their  chemical  composition,  but  also  on  the  shape  and 
size  of  the  crystals  which  make  up  the  substance  of  the 
metal.  These  crystals  in  strong,  tough  iron  and  steel  are 
so  small  that  they  can  be  detected  only  under  a  powerful 
microscope.  In  recent  years  the  microscope  has  come  to 
be  recognized  as  an  instrument  of  great  usefulness  in  the 
study  of  the  structure  of  metals.  The  effect  of  various 
kinds  of  heat-treatment  on  the  crystalline  structure  of  iron 
and  steel,  and  the  relation  between  crystalline  structure 
and  strength  and  toughness  can  be  studied  only  with  the 
aid  of  the  microscope.  Since  the  use  of  the  microscope 
has  become  general  in  testing  laboratories,  great  improve- 
ments have  been  made  in  the  strength  and  toughness  of 
iron  and  steel  by  means  of  suitable  heat-treatment:  for 
example,  the  crystalline  " grain"  of  steel  castings  has  been 
made  finer,  and  the  quality  of  steel  in  castings  has  been 
greatly  improved  by  the  development  of  annealing  proc- 
esses; the  elastic  strength  of  springs  and  automobile  parts 
has  been  greatly  raised  by  proper  tempering;  the  wearing 
properties  of  the  teeth  of  steel  gears  have  been  improved 
by  heat-treatment;  a  study  of  the  effect  of  heat-treatment 
on  the  crystalline  structure  and  properties  of  inexpensive 
grades  of  steel  has  made  it  possible  to  use  them  for  some 
machine  parts  in  place  of  more  expensive  grades. 

Crystallization  of  Pure  Iron. — Iron  like  all  solid  metals 
has  a  crystalline  structure.  This  is  shown  by  Fig.  55 
which  is  from  a  photomicrograph  of  very  nearly  pure  iron 

125 


126  MATERIALS  OF  ENGINEERING 

which  has  been  produced  by  electrolysis.1  The  metal  is 
seen  to  be  made  up  of  imperfectly  formed  crystals,  or 
rather  crystalline  grains.  Only  one  kind  of  crystals  can  be 
seen.  The  structure  is  homogeneous.  Nearly  all  pure 
metals  show  a  structure  similar  to  that  shown  in  Fig.  55. 
The  commercial  grades  of  iron  and  steel  are  not  pure 
metal,  but  contain  various  ingredients  besides  iron,  notably 
carbon.  Commercial  iron  and  steel  are  produced  from  a 
molten  state,  and  to  get  some  idea  of  the  genesis  of  their 


FIG.  55. —  Crystalline  structure  of  electrolytic  iron.    Photomicrograph  by  D.  F. 

McFarland.     Magnification  55  times. 

structure  it  will  be  convenient  to  consider  briefly  the  behav- 
ior of  liquid  mixtures  as  they  cool  and  solidify. 

Solutions,  Solid  Solutions. — When  a  substance  is  dis- 
solved in  a  liquid  (without  chemical  combination  taking 
place)  the  resulting  solution  differs  from  a  chemical  com- 
pound in  that  the  proportions  of  ingredients  are  not  fixed, 
and  it  differs  from  a  mechanical  mixture  in  the  intimacy  of 
the  union  of  the  solvent  and  the  substance  dissolved.  For 

1  Photomicrographs  are  obtained  by  polishing  the  surface  of  a  small  spec- 
imen of  the  metal  to  be  examined,  then  etching  the  polished  surface  with 
acid  to  bring  out  the  crystalline  structure,  and  finally  photographing  a  por- 
tion of  the  etched  surface  through  a  microscope.  The  process  involves 
high  degree  of  skill  in  manipulation  if  good  results  are  obtained. 


CRYSTALLINE  STRUCTURE  OF  IRON  AND  STEEL  127 

example,  in  a  salt  solution  the  intimacy  of  union  between 
salt  and  water  is  much  greater  than  in  a  mixture  of  sugar 
and  salt.  If  a  liquid  solution  is  cooled  until  solidification 
takes  place,  the  ingredients  may  act  in  one  of  three  ways: 

1.  They  may  remain  so  intimately  mixed  that  even  after 
solidifying    the    substance    shows    a    uniform    structure 
throughout,  even  when  examined  under  a  microscope.     The 
resulting  solid  is  called  a  solid  solution,  and  like  a  liquid 
solution  it  differs  from  a  chemical  compound  in  the  indefi- 
niteness   of  proportions  of  its  ingredients,   and  from  a 
mechanical  mixture  in  the  intimacy  of  mixture  of  the 
ingredients. 

2.  The  ingredients  of  the  substance  separate  as  the  solu- 
tion solidifies  and  the  resulting  solid  is  a  mechanical  mix- 
ture of  the  ingredients,  a  very  intimate  mixture  it  may  be, 
requiring  the  use  of  the  microscope  to  detect  the  two  ingre- 
dients.    In  this  second  case  the  ingredients  form  a  liquid 
solution,  but  not  a  solid  solution. 

3.  The   ingredients   may  form   a   chemical   compound. 
This  case  is  not  of  much  importance  for  iron  and  steel. 

Illustrations  of  the  Action  of  Solutions,  Eutectics. — Two 
illustrations  of  the  action  of  metal  solutions,  as  stated 
above,  will  be  given.  First,  a  solution  of  silver  in  gold  will 
be  considered.  Silver  will  dissolve  in  molten  gold  in  any 
proportion,  forming  a  liquid  solution,  and  if  the  molten 
mass  is  cooled  a  solid  solution  is  formed.  The  structure  of 
the  solid,  if  examined  under  a  microscope,  shows  only  one 
kind  of  crystal,  appearing  something  like  the  crystalline 
structure  of  pure  iron,  Fig.  55.  No  separation  of  gold 
from  silver  can  be  detected. 

Second,  a  solution  of  tin  in  molten  lead  will  be  considered. 
Tin  will  dissolve  in  molten  lead  forming  a  liquid  solution. 
If  this  solution  is  cooled  to  solidification,  separation  of  the 
constituent  metals  occurs,  and  an  examination  under  the 
microscope  shows  a  conglomerate  composed  of  fine  crystal- 
ine  grains  of  lead  and  of  tin.  If  there  is  more  than  69  per 
cent,  of  tin  in  the  liquid,  the  excess  of  tin  above  this  con- 
tent separates  out  as  a  solid  before  the  whole  mass  solidifies; 


128  MATERIALS  OF  ENGINEERING 

if  there  is  more  than  31  per  cent,  of  lead  in  the  liquid,  the 
excess  of  lead  above  this  content  separates  out  as  a  solid 
before  the  whole  mass  solidifies.  Whatever  the  propor- 
tions of  tin  and  lead  in  the  liquid  solution,  just  before  solidi- 
fication of  the  entire  mass  is  complete  the  part  remaining 
liquid  has  the  composition,  tin  69  per  cent.,  lead  31  per 
cent.,  and  the  final  solidification  of  this  particular  mixture 
takes  place  so  rapidly  that  there  is  no  time  for  the  forma- 
tion of  large  crystalline  grains,  and  as  a  consequence  the 
crystalline  grains  of  lead  and  tin  in  this  69-31  mixture  are 
small  and  very  intimately  mixed.  This  particular  mixture 
which  remains  melted  longest  and  which  solidifies  in  an 
intimate  mechanical  mixture  of  fine  grains  is  called  the 
eutectic  of  tin  and  lead.  A  solid  tin-lead  alloy  contains 
comparatively  large  crystalline  grains  of  either  tin  or  lead 
mixed  with  tin-lead  eutectic. 

A  eutectic  is  like  a  chemical  compound  in  that  its  in- 
gredients exist  in  definite  proportions,  but  under  the  mi- 
crosqope  its  structure  is  seen  to  be  a  mechanical  mixture 
of  fine  crystalline  grains  of  the  ingredients. 

The  Cooling  of  Iron-carbon  Alloys. —  All  commercial 
grades  of  iron  and  steel  are  alloys  of  iron  and  carbon.  Iron- 
carbon  alloys  in  cooling  to  solidification  combine  both  the 
actions  described  in  the  previous  paragraphs.  A  solid 
solution  of  carbon  in  iron  is  formed1  and  this  solid  solution 
acts  as  one  ingredient,  while  carbon,  if  present  in  excess  of 
about  2  per  cent.,  acts  as  a  second  ingredient,  and  these 
two  ingredients  solidify  in  a  manner  similar  to  that  de- 
scribed for  lead-tin  alloys  forming  a  eutectic.  Carbon 
will  form  a  solid  solution  in  iron  only  up  to  about  2  per 
cent,  carbon  content;  if  the  iron-carbon  alloy  has  less  than 
2  per  cent,  of  carbon,  all  the  carbon  is  taken  up  in  the  iron- 
carbon  solid  solution ;  if  the  iron-carbon  alloy  has  more  than 
2  per  cent,  carbon  the  excess  of  carbon  will  separate  out, 
normally  in  the  form  of  graphite.  Iron-carbon  alloys  con- 
taining less  than  2  per  cent,  of  carbon  are  usually  classed 
i 

1  Some  authorities  .claim  that  the  solid  solution  consists  of  iron  carbide 
(FesC)  dissolved  in  iron. 


CRYSTALLINE  STRUCTURE  OF  IRON  AND  STEEL  129 

as  steel,  and  iron-carbon  alloys  containing  more  than  2  per 
cent,  of  carbon  are  usually  classes  as  cast  iron.1 

The  Solidification  of  Cast  Iron. — The  eutectic  for  an 
iron-carbon  alloy  containing  more  than  about  2  per  cent, 
of  free  carbon  has  for  its  ingredients  4.3  per  cent,  of  free 
carbon,  normally  in  the  form  of  graphite,  and  95.7  per  cent. 
of  a  solid  solution  of  carbon  in  iron.  If  the  alloy  as  a  whole 
contains  more  than  2  per  cent,  of  carbon  and  less  than  4.3 
per  cent,  of  free  carbon  the  excess  of  " iron-carbon  solid 
solution"  solidifies  first,  until  the  carbon  content  of  the 
remaining  liquid  reaches  4.3  per  cent.,  when  the  eutectic 
solidifies.  The  resulting  solid  consists  of  crystalline  grains 
of  a  solid  solution  of  iron  mixed  with  crystalline  grains  of 
the  iron-carbon  eutectic.  If  the  alloy  contains  more  than 
4.3  per  cent,  of  free  carbon,  the  excess  ingredient  is  carbon, 
which  solidifies  from  the  liquid  in  the  form  of  graphite. 
The  resulting  solid  consists  normally  of  flakes  of  graphite 
and  crystalline  grains  of  the  iron-carbon  eutectic.  In  the 
above  it  has  been  stated  that  normally  carbon  separates 
from  the  solution  in  the  form  of  graphite.  This  is  true  if 
the  cooling  takes  place  slowly.  If  the  cooling  takes  place 
rapidly  there  is  not  time  enough  for  the  graphite  flakes  to 
form,  and  the  carbon  is  present  in  the  form  of  "  combined 
carbon"  (iron  carbide,  Fe3C),  which  makes  the  resulting 
solid  hard  and  brittle.  White  cast  iron  has  its  carbon  in 
the  form  of  combined  carbon  rather  than  in  the  form  of 
graphite  (see  Fig.  43,  page  111). 

The  Cooling  of  Steel  to  Solidification  and  after  Solidifi- 
cation.— As  steel  is  an  iron-carbon  alloy  containing  less 
than  2  per  cent,  of  carbon  it  solidifies  as  a  solid  solution 
of  carbon  in  iron.  The  remarkable  thing  about  steel  is 
that  the  changes  in  its  crystalline  structure  do  not  cease 
when  solidification  occurs,  but  continue  to  take  place  as 
the  solid  steel  cools  until  the  temperature  is  about  690°C. 
(1,274°F.)  far  below  the  temperature  of  solidification  of 
molten  steel. 

1  The  line  of  demarcation  between  steel  and  cast  iron  is  given  different 
values  by  different  authorities,  ranging  from  1.7  to  2.2  per  cent. 


130 


MATERIALS  OF  ENGINEERING 


At  the  temperature  of  solidification  steel  is  a  solid  solu- 
tion of  carbon  in  iron,  but  as  cooling  goes  on  this  solid 
solution  normally  breaks  up  into  two  ingredients,  carbide 
of  iron  called  cementite,  and  pure  iron  called  ferrite.  If 
the  carbon  content  of  the  steel  is  greater  than  0.90  per  cent., 
cementite  will  separate  out  before  the  steel  takes  its  final 
crystalline  form;  if  the  carbon  content  is  less  than  0.90 
per  cent,  ferrite  will  separate  out.  In  either  case  the  last 
part  of  the  steel  to  reach  its  final  crystalline  form  has  a 
carbon  content  of  0.90  per  cent.  This  is  called  the  eutec- 


(Courtesy  of  the  Wy man-Gordon  Co.  and  the  Aluminum  Castings  Co.) 
a.  Steel  with  0.32  per  cent,  carbon     6.  Pearlite.     Photomicrograph  by  R.  S. 
content.     Photomicrograph    by    J.     H.         Archer.     Magnification  650  times. 
Nelson.     Magnification  75  times. 
FIG.  56. — Crystalline  structure   of  ferrite-pearlite  steel    (hypoeutectoid   steel). 

toid  of  steel.  The  properties  of  the  eutectoid  are  like  those 
of  a  eutectic,  but  the  eutectoic  is  formed  from  a  solid 
not  from  a  liquid.  The  eutectoid  of  steel  is  called  pearlite 
and  consists  of  very  fine  intimately  mixed  grains  of  ferrite 
and  cementite.  (See  Fig.  566.) 

Steel  with  a  carbon  content  of  less  than  0.90  per  cent., 
commonly  called  hypoeutectoid  steel,  is  normally  made  up 
of  crystalline  grains  of  ferrite  and  crystalline  grains  of 
pearlite.  Fig.  56  is  from  a  photomicrograph  of  such  steel; 
the  light  portions  are  ferrite,  and  the  dark  portions  are 


CRYSTALLINE  STRUCTURE  OF  IRON  AND  STEFL  131 

pearlite.  Under  a  very  high  magnification  the  structure 
of  pearlite  would  be  seen  to  be  made  up  of  an  intimate 
mixture  of  very  fine  grains  of  ferrite  and  of  cementite. 
(See  Fig.  566.) 

Steel  with  a  carbon  content  of  more  than  0.90  per  cent., 
commonly  called  hypereutectoid  steel,  if  allowed  to  cool 
slowly  is  made  up  of  crystalline  grains  of  cementite  and 
crystalline  grains  of  pearlite.  Fig.  57  is  from  a  photo- 
micrograph of  such  steel.  The  light  portions  are  cementite, 
and  the  dark  portions  are  pearlite. 


FIG.  57. — Crystalline  structure  of  annealed  cementite-pearlite  steel    (hyper- 
eutectoid steel).     Photomicrograph  by  H.  T.  Manuel.     Magnification  265  times. 

In  slowly  cooled  steel  the  principal  ingredients  are  ferrite 
and  pearlite,  or  cementite  and  pearlite.  Under  the  micro- 
scope other  ingredients  can  be  detected  by  the  trained 
observer.  Among  them  are  slag,  which  is  always  present 
in  wrought  iron  (see  Fig.  30,  page  84)  and  is  sometimes 
present  in  small  quantities  in  steel;  manganese  sulphide, 
globules  of  which  are  sometimes  mixed  in  the  structure  of 
the  steel;  and  iron  oxide,  due  to  the  direct  oxidation  of  the 
iron  during  the  removal  of  impurities. 

The  Critical  Temperature  of  Steel,  the  Recalescence 
Point. — As  steel  is  heated  or  cooled  the  change  in  its  tem- 
perature does  not  proceed  regularly  but  shows  sudden  vari- 
ations at  certain  stages.  The  most  important  stage  is  that 
at  which  the  eutectoid  (pearlite)  forms,  which  occurs  at  a 


132  MATERIALS  OF  ENGINEERING 

temperature  of  about  690°C.  (1,274°F.).  When  this  tem- 
perature is  reached  in  cooling  steel,  there  is  a  sudden  giving 
off  of  heat,  the  cooling  process  is  momentarily  checked,  and 
if  the  steel  is  in  a  dark  room  it  suddenly  is  seen  to  glow 
more  vigorously.  This  temperature  is  called  a  critical 
temperature  or  the  steel  is  said  to  be  at  the  recalescence 
point.  The  structure  of  the  steel  has  not  reached  a  stable 
condition  until  this  point  is  reached,  and  normally  is  in 
a  stable  condition  after  the  recalescence  point  is  passed. 

The  changes  taking  place  at  the  recalescence  point  re- 
quire some  time  for  their  completion,  and  if  the  cooling  is 
rapid  the  normal  structure  of  the  steel  may  not  be  devel- 
oped. In  the  case  of  low-carbon  steels  the  principal  in- 
gredient is  ferrite  and  very  little  variation  from  the  normal 
can  be  brought  about  by  sudden  cooling.  In  the  case  of 
steel  with  a  carbon  content  greater  than  about  0.25  per 
cent.,  sudden  cooling  from  above  the  recalescence  point 
does  not  give  time  for  complete  formation  of  the  eutectoid 
(pearlite)  and  some  of  the  solid  solution  of  carbon  in  iron 
seems  suddenly  frozen — or  it  may  be  that  the  iron  itself 
exists  in  a  different  (allotropic)  form.  The  structure  de- 
veloped in  steel  as  it  cools  depends  on  the  rapidity  of  cool- 
ing. If  the  cooling  of  high  carbon  steel  is  carried  on  with 
extreme  rapidity — e.g.  by  cooling  in  brine  at  0°  Fahr. 
— a  considerable  portion  of  the  steel  is  left  in  the  form  of 
a  solid  solution  of  carbon  (or  iron  carbide)  in  iron.  This 
solid  solution  is  known  as  austenite.  Austenite  is  very 
rarely  an  ingredient  in  commercial  steels,  except  a  very 
few  special  alloy  steels  in  which  some  alloying  ingredient 
(e.g.  manganese)  acts  to  retard  very  markedly  the  struct- 
ural transformation  of  steel  as  it  cools. 

If  the  cooling  of  high-carbon  steel  takes  place  at  a 
somewhat  slower  rate  than  is  the  case  for  cooling  in  cold 
brine,  e.g.  by  cooling  in  water  at  room  temperature — a 
structure  of  needle-like  crystals  is  developed,  known  as 
martensite.  Fig.  58  shows  this  structure.  Steel  in  the 
form  of  martensite  is  very  hard  and  very  brittle.  Such 
steel  is  suitable  for  very  sharp-edged  cutting  tools,  such 


CRYSTALLINE  STRUCTURE  OF  IRON  AND  STEEL  133 

as  razor  blades,  but  is  not  suitable  for  stress-carrying  mem- 
bers of  machines  or  structures  on  account  of  its  brittleness. 


FIG.  58. — Martensite,  characteristic  crystalline  structure  of  very  suddenly 
cooled  hypereutectoid  steel.  Photomicrograph  by  H.  T.  Manuel,  Univ.  of  Illinois. 
Magnification  265  times. 

With  still  slower  cooling  the  characteristic  structure 
developed  is  known  as  troostite,  shown  in  Fig.  59a.  Troos- 


a.  Troostite.     Photomicrograph    by    J.     b.  Sorbite.      Photomicrograph  by  R.  S. 
W.  Harsch.     Magnification  350  times.  Archer.     Magnification  75  times. 

FIG.  59. — Crystalline  structure  of  steel  with  varying  rates  of  cooling  (Courtesy 

of  Univ.  of  III.  Metallographic  Lab.  and  the  Aluminum  Castings  Co.) 
Fig.  57,  Fig.  58,  and  Fig.  59  all  give  photomicrographs  of  hypereutectoid 
steel.  As  the  suddenness  of  cooling  increases  the  structure  obtained  is  succes- 
sively cementite-pearlite  (Fig.  57),  sorbite  (Fig.  596),  troostite  (Fig.  59a),  and 
martensite  (Fig.  58).  The  sorbitic  structure  with  its  extremely  fine  grain  is 
supposed  to  give  the  toughest  steel,  and  to  give  maximum  resistance  to  repeated 
stress. 

tite  can  be  identified  by  the  fact  that  in  the  process  of 
etching  it  assumes  a  very  dark  color.     Troostite  is  slightly 


134  MATERIALS  OF  ENGINEERING 

softer,  slightly  weaker, .  and  slightly  more  ductile  than 
martensite.  The  structure  of  hardened  steel  used  in  cut- 
ting tools  and  machine  parts  is  usually  made  up  of  marten- 
site,  troostite,  or  a  mixture  of  the  two. 

With  still  slower  cooling — e.g.,  cooling  in  an  oil  bath  at 
a  temperature  somewhat  above  room  temperature — steel 
assumes  the  structure  known  as  sorbite,  which  is  shown  in 
Fig.  596.  Sorbite  seems  to  be  an  intermediate  structure 
between  hardened  steel  (troostite)  and  annealed  steel 
(pearlite).  The  sorbite  structure  seems  to  yield  steel  of 
very  high  strength  and  a  fair  degree  of  ductility, — a  tough 
steel.  Sorbite  is  generally  regarded  as  the  ideal  structure 
for  medium  and  high-carbon  steel  to  be  used  for  stress- 
carrying  parts  of  machines. 

With  very  slow  cooling  (annealing)  the  structure  of  steel 
consists  of  pearlite  and  ferrite  for  hypoeutectoid  steels 
(see  Fig.  56)  and  of  pearlite  and  cementite  for  hypereutec- 
toid  steels  (see  Fig.  57).  For  steel  with  a  carbon  content 
less  than  about  0.25  per  cent,  the  changes  of  structure  dur- 
ing cooling  take  place  so  rapidly  that  even  with  water-cooled 
steel  the  structure  found  is  practically  always  ferrite  and 
pearlite.  Carbon  in  steel  acts  as  a  brake  to  slow  up  the 
changes  from  one  stage  to  another. 

Tempering  Steel. — If  steel  is  suddenly  cooled,  leaving 
its  structure  martensite,  troostite,  or  sorbite  the  structure 
is  a  state  of  unstable  equilibrium,  and  will  start  to  change  to 
pearlite  if  heated  to  temperatures  somewhat  below  the 
recalescence  point.  By  heating  to  temperatures  more  or 
less  closely  approaching  the  critical  temperature  the 
degree  of  transformation  can  be  regulated,  and  with  it 
the  hardness,  strength  and  ductility  of  the  steel.  Mar- 
tensitic  steel  can  be  tempered  to  troostitic  steel  producing 
a  steel  somewhat  less  brittle,  or  to  sorbitic  steel,  pro- 
ducing a  steel  combining  a  high  degree  of  strength  and 
ductility. 

The  object  of  tempering  or  "heat  treating"  steeljmay 
be  either  to  produce  a  desired  degree  of  hardness,  as  in 
cutting  tools,  or  to  produce  a  desired  combination  of 


CRYSTALLINE  STRUCTURE  OF  IRON  AND  STEEL  135 

strength  and  ductility,  as  in  the  case  of  axles  and  crank- 
pins  for  automobiles. 

The  technique  of  heat-treating  steel  to  produce  a  desired 
quality  is  very  complex,  and  can  not  be  treated  in  this 
book.  It  may  be  noted,  however,  that  a  study  of  the  effect 
of  heat-treatment  on  crystalline  structure  and  resulting 
strength,  hardness,  and  ductility  has  in  many  cases  made 
possible  the  use  of  a  heat-treated  inexpensive  grade  of 
steel  in  place  of  an  expensive  grade. 

The  effectiveness  of  certain  ingredients  in  steel,  notably 
manganese,  in  making  it  hard  even  when  cooled  in  air  is 
due  mainly  to  their  power  to  retard  the  formation  of  the 
eutectoid  as  steel  cools,  so  that  even  if  the  cooling  proceeds 
slowly  in  air  the  steel  is  still  hard  when  it  reaches  atmos- 
pheric temperature. 

Grain  Size  of  Iron  and  Steel. — If  iron  or  steel  is  cooled 
very  slowly  and  quietly  past  the  recalescence  point,  there 
is  given  time  for  the  formation  of  a  coarse  crystalline  struc- 
ture, and  owing  to  the  large  planes  of  cleavage  between 
grains  the  steel  tends  to  be  brittle.  If  the  steel  is  cooled 
rapidly  or  is  hammered  while  being  cooled  the  resulting 
structure  will  be  fine-grained,  and  the  material  will  be  im- 
proved in  ductility.  If  coarse-grained  steel  is  heated 
slightly  above  the  recalescence  point  the  eutectoid  struc- 
ture will  be  broken  up  and  the  carbon  will  go  into  solid 
solution  in  iron.  If  the  metal  is  then  cooled,  the  recales- 
cence point  will  soon  be  reached,  before  there  has  been  time 
for  the  formation  of  large  grains,  and  the  resulting  steel 
will  be  fine-grained.  This  process  has  been  called  the  re- 
fining of  steel  by  heat-treatment.  Fig.  60  shows  changes 
in  grain  size  of  steel  caused  by  cooling  in  different  ways. 
The  change  in  grain  size  may  be  brought  about  in  any 
grade  of  steel,  though  high-carbon  steels  are  more  sensitive 
to  change  of  grain  size  by  heat-treatment  than  are  low-car- 
bon steels.  The  change  of  grain  size  by  proper  heat-treat- 
ment is  widely  utilized  by  steel  makers  and  steel  users. 
Steel  castings  are,  of  necessity,  slowly  cooled  in  their  molds 
from  the  temperature  of  melted  steel  to  a  temperature  but 


136 


MATERIALS  OF  ENGINEERING 


little  above  atmosphere.  The  resulting  grain  structure  is 
coarse,  and  the  steel  is  brittle;  great  improvement  in  the 
quality  of  steel  castings  can  be  brought  about  by  annealing. 
If  two  pieces  of  iron  or  steel  are  welded  together,  it  is  nec- 
essary first  to  heat  the  ends  to  be  welded  to  a  temperature 
almost  that  of  fusion.  This  high  heating  causes  coarse 
crystalline  structure  in  the  material  near  the  weld.  A 
welded  joint  can  usually  be  made  tougher  by  annealing 
after  the  weld  is  made. 

Steel  heated  to  very  high  temperatures  in  the  presence 
of  air  is  sometimes  actually  " burned/'  and  under  the  micro- 
scope is  seen  to  contain  iron  oxide  in  appreciable  quantity. 


a  b  c  d  e 

a.  Steel  as  rolled. 
b  Heated  to  1200  degrees  C.  for  2  hours,  cooled  in  furnace. 

c.  Heated  to  1000  degrees  C.  for  6  hours,  cooled  in  furnace. 

d.  Heated  to  900  degrees  C.,  cooled  in  air. 

e.  Heated  to    725  degrees  C.,  quenched  in  water,  reheated  to  650  degrees  C., 
cooled  in  air;  sorbitic  structure. 

FIG.  60. — Crystalline  structure  of  0.40  per  cent,  carbon  steel  with  various  heat 
treatments.  Photomicrographs  by  Henry  Daubet,  Univ.  of  III.  Magnification  55 
times. 

Annealing  will  not  restore  the  quality  of  such  steel;  it 
can  be  restored  only  by  remelting  under  a  deoxidizing 
slag. 

The  grain  size  of  steel  may  be  kept  small  by  hammering 
or  rolling  the  steel  as  cooling  occurs.  Quiet  and  slow  cool- 
ing are  necessary  for  the  formation  of  large  crystals,  and  as 
large  crystals  are  undesirable  in  structural  metals,  hammer- 
ing or  rolling  the  metal  as  it  cools  improves  the  quality. 
This  improvement  in  quality  is  well  illustrated  by  the  in- 
creased toughness  given  to  welded  joints  by  hammering 


CRYSTALLINE  STRUCTURE  OF  IRON  AND  STEEL  137 

them  as  they  cool,  also  by  the  superior  toughness  of  rolled 
steel  as  compared  with  steel  castings. 

Annealing  Steel  to  Remove  the  Effects  of  Overstress.— 
In  service  machine  parts  are  occasionally  overstressed,  and 
if  made  of  ductile  metal  are  "  cold- worked "  by  this  over- 
stressing  with  the  result  that  their  resistance  to  subsequent 
impact  and  to  repeated  stress  is  diminished.  Heating  the 
metal  slightly  above  the  critical  temperature  removes  the 
effects  of  such  overstress.  Iron  and  steel  chains  are  fre- 
quently subjected  to  overstress  when  the  links  "kink" 
or  a  sudden  impact  load  comes  on  a  chain,  and  some 
large  users  of  chain  anneal  their  chains  at  regular  intervals. 
Such  annealing  should  be  done  in  a  furnace  where  the 
temperature  can  be  controlled  and  kept  uniform.  Merely 
heating  a  chain  in  a  forge  fire  usually  results  in  overheating 
some  links  and  underheating  others,  with  damage  rather 
than  improvement  to  the  quality  of  the  chain  as  a  whole. 
The  same  difficulty  would  be  met  in  annealing  any  machine 
part  in  an  open  fire. 

Whether  annealing  can  be  counted  on  to  weld  up  the 
microscopic  cracks  caused  by  fatigue  of  steel  under  re- 
peated stress  is  an  unsolved  question. 

The  Welding  of  Steel;  Types  of  Welds.— Welds  in 
steel,  and  in  other  metals,  may  be  divided  into  two  classes : 
(1)  welds  made  at  a  plastic  heat,  and  (2)  welds  made  by 
actual  fusion  of  the  adjacent  metal  in  the  parts  joined. 
Plastic  welding  is  accomplished  by  bringing  the  edges  to  be 
joined  up  to  a  temperature  a  little  below  fusion  and  then 
pressing  or  hammering  them  together.  Wrought  iron  can 
be  readily  welded  by  plastic  welding,  soft  steel  can  be 
welded  without  much  difficulty  and  high  carbon  steel 
can  be  thus  welded  only  by  a  skilled  workman.  Cast 
iron  pieces  cannot  be  joined  by  plastic  welding. 

Plastic  welding  is  sometimes  done  in  a  forge  fire,  as  is 
the  usual  case  for  the  welding  of  rods  or  of  chain  links,  or 
the  heat  may  be  supplied  by  means  of  an  electric  current 
passing  through  the  parts  to  be  welded.  Special  welding 
machines  using  electric  current  are  used  for  welding  rods, 


138 


MATERIALS  OF  ENGINEERING 


chain  links,  and  other  machine  parts;  these  are  to  be 
distinguished  from  the  machines  for  making  electric  welds 
by  fusion  of  the  metal,  which  are  noted  a  little  later. 
Spot  welding  is  a  special  method  of  plastic  welding  by  the 
use  of  electric  current,  and  its  operation  is  shown  in  prin- 
ciple in  Fig.  61.  Alternating  current  is  supplied  at  low 
voltage  by  a  transformer  T  and  passes  through  electrodes 
CD  and  the  "pieces  EF  to  be  joined.  Pressure  is  applied 
to  the  electrodes,  and  sufficient  heat  is  generated  in  the 
metal  to  bring  a  small  spot  to  a  plastic  welding  heat,  and 
thus  to  "tack"  the  pieces  together.  Spot  welding  is  used 


T       /Low  Volfage 

Pressure 


FIG.  61. — Diagram  of  spot- welding  apparatus. 

for  fastening  together  pieces  which  do  not  have  to  transmit 
heavy  stress,  or  for  "tacking"  together  pieces  which  are 
to  be  more  thoroughly  welded  by  a  fusion  process  of  welding 
as  described  in  the  next  paragraph. 

Fusion  Welding. — If  the  edges  of  two  adjacent  pieces 
of  metal  are  heated  to  a  temperature  of  fusion,  and  then 
allowed  to  cool  the  process  is  known  as  fusion  or  autogenous 
welding.  The  high  localized  temperatures  required  may 
be  produced  by  (1)  the  heat  of  a  flame  of  gas,  usually 
acetylene,  burning  in  a  stream  of  pure  oxygen,  (2)  the  igni- 
tion of  a  mixture  of  iron  oxide  and  aluminum  (the  thermit 
process),  and  (3)  the  use  of  the  electric  arc. 

In  welding  with  an  oxyacetylene  torch  the  oxygen  and 
the  acetylene  are  fed  through  a  blow  pipe  and  ignited  at  its 
tip.  The  flame  has  a  temperature  of  approximately 


CRYSTALLINE  STRUCTURE  OF  IRON  AND  STEEL.1M 

500Q°F.  and  a  narrow  strip  of  metal  at  the  junction  of 
the  parts  is  fused,  uniting  the  parts.  Usually  additional 
metal  is  added  to  the  joint  by  melting  in  a  small  steel  or 
iron  rod.  Wrought  iron,  steel,  cast  iron,  copper,  brass, 
aluminum,  and  numerous  alloys  of  those  metals  are  welded 
by  the  oxyacetylene  process.  Hydrogen  and  other  fuel 
gases  are  sometimes  used  for  welding,  but  acetylene  is 
by  far  the  most  common  gas  used  for  that  purpose.  If  an 
excess  of  oxygen  is  supplied  to  the  torch  it  becomes  a  very 
effective  cutting  tool,  or,  more  correctly  a  burning  tool, 
for  wrought  iron  or  steel.  In  the  presence  of  excess  of 
pure  oxygen  heated  iron  or  steel  burns,  and  an  oxyacetylene 
cutting  torch  burns  a  narrow  gash  when  applied  to  a  piece 
of  steel  or  iron.  The  oxyacetylene  cutting  torch  is  widely 
used  to  cut  up  steel  scrap. 

In  the  thermit  process  of  welding  a  specially  prepared 
mixture  of  aluminum  and  iron  oxide  is  placed  in  a  crucible 
and  ignited.  The  heat  of  the  resulting  combustion  raises 
the  temperature  of  the  mass  to  about  4800°F.,  and  there  is 
produced  superheated  melted  iron  which  is  poured  round 
the  joint  to  be  welded,  and  which  forms  a  solid  casting  at 
the  joint. 

In  the  electric  arc  process  of  welding  an  arc  is  drawn 
between  the  metal  to  be  welded  and  an  electrode  (usually 
a  steel  or  iron  rod)  held  in  the  hand  of  the  operator.  The 
heat  of  the  arc  melts  the  end  of  the  electrode  and  also 
melts  metal  at  the  surfaces  to  be  joined.  The  electric 
current  supplied  may  be  either  direct  or  alternating. 

Applications  of  Different  Types  of  Welds. — Forge  weld- 
ing is  used  for  general  blacksmith  work,  for  chain  making, 
and  for  repair  work.  Forge-made  welds  are  hammered 
as  they  cool,  and  if  the  heating  has  been  properly  done 
the  resulting  joint  has  a  fine  crystalline  structure  and  is 
as  strong  as  the  original  metal.  However,  the  difficulty  of 
judging  and  controlling  temperatures  under  ordinary 
shop  conditions  is  so  great  that  forge  welds,  in  general,  can 
not  be  counted  on  for  more  than  50  per  cent,  of  the  strength 
of  the  material  welded. 


140  MATERIALS  OF  ENGINEERING 

Plastic  welding  with  an  electric  current  to  supply  heat 
is  used  in  special  manufacturing  operations,  such  as  welding 
valve  stems  to  valve  seats,  chain  manufacture,  etc.  This 
process  permits  close  control  of  temperature  and  in  some 
cases  pressure  or  hammering  as  the  weld  cools,  with  the 
result  of  producing  a  fine  crystalline  structure  throughout 
the  joint. 

Oxyacetylene  welding  is  especially  applicable  to  thin 
plates  of  metal.  The  joint  produced  is  a  casting  and  has 
the  rather  coarse  crystalline  structure  characteristic  of 
castings,  and  has  corresponding  physical  properties.  An- 
nealing somewhat  improves  this  structure.  The  com- 
monest weakness  in  oxyacetylene  welded  joints  arises 
from  failure  to  fuse  thoroughly  the  metals  at  the  joint, 
leaving  a  zone  of  unwelded  metal  at  the  middle  of  the  thick- 
ness of  the  plates.  Oxyacetylene  welding  is  widely  used 
for  repair  work,  and  as  it  is  a  fusion  process  cast  metals 
can  be  welded  as  well  as  rolled  metals.  The  very  high 
temperature  applied  to  a  narrow  area  often  produces 
heavy  shrinkage  strains  in  the  cooled  parts,  and  in  welding 
complicated  shapes  it  is  frequently  necessary  to  preheat  the 
pieces  to  minimize  such  strains.  The  material  in  carefully 
made  oxyacetylene  welds  has  about  80  per  cent,  of  the 
strength  of  the  material  welded.  There  is,  however,  always 
danger  of  overheating  the  metal  adjacent  to  the  weld,  and 
under  ordinary  repair  shop  conditions  it  would  hardly  be 
safe  to  count  on  an  efficiency  of  an  oxyacetylene  weld 
greater  than  50  per  cent. 

The  thermit  process  is  especially  adaptable  to  repair 
work  on  heavy  sections  and  to  heavy  welds  in  the  field. 
Welding  electric  railway  rails,  repairing  broken  punch 
frames,  cracked  locomotive  frames,  etc.  are  examples  of 
the  .use  of  the  thermit  process.  The  thermit  process 
produces  cast  metal  at  the  welded  joint.  The  process  is 
patented. 

Electric  arc  welding  requires  an  installation  of  trans- 
formers or  motor-generators  to  produce  current,  and  neces- 
sitates an  available  source  of  electric  power.  It  is  usually 


CRYSTALLINE  STRUCTURE  OF  IRON  AND  STEEL  141 


used  under  manufacturing  plant  conditions  rather  than 
under  repair  shop  conditions.  The  joint  produced  is  a 
casting  and  is  similar  in  its  characteristics  to  an  oxy- 
acetylene  welded  joint.  It  is  a  disputed  question  whether 
electric  arc  welds  or  oxyacetylene  welds  are  the  better. 
The  electric  arc  process  is  somewhat  quicker  than  the 
oxyacetylene  and,  where  a  large  amount  of  welding  is 
to  be  done,  somewhat  cheaper.  For  manufacturing  the 
electric  arc  process  is  used  for  somewhat  heavier  work 
than  the  oxyacetylene,  though  on  account  of  the  porta- 
bility of  the  outfit  the  oxyacetylene  process  is  used  for 
heavy  repair  work. 

Strength  of  Steel  and  Other  Metals  under  High  Tempera- 
tures.— It  is  a  matter  of  general  knowledge  that  iron  and 


Temperature  Facto  r(F") 

P  <0  0  0  - 

_0  ro  4>  <y  o>  0 

X 

v 

\ 

For  any  Temperature  (t) 
•the  ultima  fa  Tensile  Strength 
of  Metal  '(Strength  at 
Ordinary  Temperature)*. 
(F)  corresponding  to  (t) 

X 

^V 

\ 

A 

S 

\\ 

N, 

<X 

X 

^s 

X 

^v. 

\ 

x\ 

^ 

& 

N, 

\\ 

\\ 

"% 

Vv 

^X 

N^-X 

X 

^\ 

V 

\ 

X 

,  \ 

X 

KX)             200             300              400              500              600             700             800             90 
Temperature,  Degrees  Centigrade 

3Z    100 


500  1000 

Temperature,  Degrees  Fahrenheit. 


FIG.  62. — Effect  of  temperature  on  the  strength  of  steel  and  of  other  metals. 

steel  lose  nearly  all  their  stiffness  and  strength  at  red  heat. 
A  summary  of  experiments  on  the  strength  of  various  grades 
of  steel  shows  that  the  ultimate  increases  very  slightly 
up  to  about  500°F.  (260°C.)  above  which  the  ultimate 
diminishes  approximately  in  proportion  to  the  temperature. 
The  material  loses  practically  all  its  strength  at  about 
1,600°F.  (870°C.).  The  change  of  value  of  proportional 
limit  under  increase  in  temperature  has  not  been  so  thor- 


142  MATERIALS  OF  ENGINEERING 

oughly  studied,  but  such  test  data  as  are  available  indicate 
a  regular  diminution  of  proportional  limit  from  atmos- 
pheric temperatures  to  a  temperature  of  about  1,600°F., 
where  the  proportional  limit  becomes  zero. 

Fig.  62,  which  is  based  on  test  data  given  by  Howard  of 
the  U.  S.  Interstate  Commerce  Commission,  by  Ludwik 
of  the  Royal  Technical  High  School  of  Vienna,  by  Rudeloff 
and  by  Bach  shows  graphically  the  reduction  of  ultimate 
strength  of  various  metals  which  should  be  allowed  for 
different  temperatures. 

Selected  References  for  Further  Study 

MELLOR:  "The  Crystallization  of  Iron  and  Steel,"  London,  1905.  A  con- 
cise elementary  text. 

WILLIAMS:  "Principles  of  Metallography,"  New  York,  1920.  A  concise, 
elementary  text. 

STOUGHTON:  "The  Metallurgy  of  Iron  and  Steel,"  New  York,  1911,  Chaps. 
X,  XI,  XII,  XIV,  XVIII,  XIX. 

DESCH:  "Metallography,"  London,  1910.  A  concise  text  written  from  the 
viewpoint  of  the  physical  chemist. 

SAUVEUR:  "The  Metallography  and  Heat  Treatment  of  Iron  and  Steel," 
Cambridge,  Mass.,  1916.  A  comprehensive  text  by  an  American 
metallurgist. 

HOWE:  "Iron,  Steel  and  Other  Alloys,"  New  York,  1906.  A  comprehen- 
sive text  by  the  leading  American  authority  on  the  metallurgy  of  iron 
and  steel. 

CONE:  Use  of  the  Microscope  in  the  Study  of  Metals,  Iron  Age,  Oct.  16, 
1913. 

ABBOTT:  Modern  Steels  and  Their  Heat  Treatment,  Journal  of  the  Franklin 
Institute,  Jan.  14,  1915. 

HOBART:  "Welding  Mild  Steel,"  Trans.  Am.  Inst.  Mining  Engrs.,  Feb., 
1919. 

HART:  "Welding,  Theory,  Practice,  Apparatus,  and  Tests,"  New  York,  1910. 
A  concise  general  text. 


CHAPTER  XIII 

THE   EFFECT  OF  VARIOUS  INGREDIENTS  ON  THE 
PROPERTIES  OF  IRON  AND  STEEL;  CORROSION 

The  Importance  of  Chemical  Composition  of  Iron  and 
Steel.— In  Chaps.  X,  XI  and  XII  the  effects  of  heat- 
treatment  and  mechanical  treatment  of  iron  and  steel 
were  discussed.  Of  equal  if  not  of  greater  importance  is 
the  effect  on  the  strength,  hardness,  ductility,  and  toughness 
of  iron  and  steel  produced  by  the  presence  of  various  ingre- 
dients. So  many  ingredients  are  present  in  the  various 
grades  of  commercial  iron  and  steel  that  anything  like  a 
systematic  tabulation  of  their  effects  is  impossible.  This 
chapter  will  discuss  in  a  general  way  the  effects  of  the  more 
common  ingredients  found  in  steel. 

Commercial  Pure  Iron. — Swedish  wrought  iron  and  very 
low-carbon  open-hearth  steel  are  the  commercial  prod- 
ucts which  are  nearest  to  pure  iron.  Experimentally, 
iron  of  still  higher  purity  has  been  produced  by  electrolysis. 
Pure  iron  has  a  tensile  strength  of  about  40,000  Ib.  per 
square  inch,  is  very  ductile,  and  as  compared  with  the  com- 
mercial grades  of  steel  is  very  soft. 

Carbon. — Carbon  up  to  about  1.25  per  cent,  increases 
the  strength  of  iron,  and  the  increase  is  approximately  pro- 
portional to  the  carbon  content.  Carbon  in  chemical 
combination  with  iron  in  the  form  of  cementite  increases 
the  hardness  and  lowers  the  ductility.  An  alloy  of  iron 
and  carbon  containing  less  than  about  2  per  cent,  of  carbon 
is  called  steel  (except  the  product  of  the  puddling  process 
which  is  wrought  iron);  an  alloy  of  iron  and  carbon  con- 
taining more  than  about  2  per  cent,  of  carbon  is  called 
cast  iron.  The  reason  for  this  distinction  is  discussed 
in  Chap.  XII. 

143 


144  MATERIALS  OF  ENGINEERING 

The  carbon  contents  of  the  common  varieties  of  steel  are : 

Per  cent. 

Soft  steel 0.05-0. 15 

Structural  steel 0. 15-0. 25 

Medium  steel  for  forgings 0. 20-0.40 

Rail  steel 0.35-0.55 

Spring  steel 0.80-1 . 10 

Carbon  steel  for  cutting  tools 0. 60-1 . 50 

In  cast  iron  the  carbon  may  be  present  either  as  com- 
bined carbon  or  as  graphite.  In  general,  slow  cooling  of 
the  iron  from  a  molten  state  causes  a  precipitation  of  the 
carbon  as  flakes  of  graphite  and  "gray  "  iron  is  the  product : 
quick  cooling  tends  to  cause  the  retention  in  combination 
of  a  high  percentage  of  carbon,  and  the  product  is  " white" 
iron.  Up  to  about  1.25  per  cent.,  combined  carbon  in- 
creases the  strength  of  cast  iron,  while  graphite  lowers 
the  strength  but  makes  cast  iron  soft  and  readily  machined. 
The  flakes  of  graphite  as  they  are  precipitated  from  the 
molten  iron  take  up  space  between  the  crystals  of  iron, 
causing  a  tendency  for  the  iron  to  expand  in  the  mold  and 
diminishing  the  final  shrinkage  of  the  iron  which  takes  place 
as  the  iron  cools,  and  which  for  gray  cast  iron  amounts  to 
about  ^  in.  per  foot. 

Silicon. — Ordinary  steels  contain  less  than  0.20  per  cent, 
of  silicon,  and  this  amount  has  no  appreciable  effect  on  the 
strength  or  the  ductility.  Special  silicon  steel  contains 
much  larger  percentages  of  silicon,  usually  from  1.0  to  4.0 
per  cent.  A  silicon  steel  containing  low  carbon  content 
and  about  1.9  per  cent,  of  silicon  has  recently  been  used  as  a 
structural  steel  of  high  strength.  A  silicon  steel  with  about 
0.50  per  cent,  carbon,  2.0  per  cent,  silicon,  and  0.70  per  cent, 
manganese  has  been  used  for  leaf  springs  for  automobiles 
and  carriages.  This  last-named  alloy  is  sometimes  called 
silico-manganese  steel.  Steel  with  about  4  per  cent,  sili- 
con and  a  very  low  carbon  content  is  used  for  transformer 
cores  on  account  of  its  very  high  magnetic  permeability. 
Silicon  tends  to  give  a  bath  of  molten  steel  an  acid  reac- 
tion and  hence  steel  made  in  a  basic  furnace  always  has 
a  low  silicon  content. 


THE  EFFECT  OF  VARIOUS  INGREDIENTS       145 

Phosphorus. —Phosphorus  is  known  as  "the  steel  maker's 
bane."  Small  amounts  increase  the  strength  of  steel,  but 
increase  its  brittleness  especially  under  cold  weather  tem- 
peratures (see  Fig.  63).  Steel  which  is  brittle  when  cold 
is  called  "cold  short."  Phosphorus  is  especially  dangerous 
in  railroad  rails  which  are  exposed  to  severe  shocks  in 
service,  and  which,  in  winter,  are  exposed  to  low  tempera- 
tures. For  most  purposes  not  more  than  0.05  per  cent,  of 
phosphorus  is  allowable  in  steel,  except  for  thin  rolled 
plates  in  which  experience  has  shown  that  the  presence 
of  phosphorus  makes  hot-rolling  easier. 


UHU 

030 

0.20 
0.10 

0 

\ 

y 

\ 

•\ 

\ 

\ 

V 

.0          10          ZO         30        40 
Elongation  in  8  Inches  Per  Cent 
FIG.  63. — Effect  of  phosphorus  on  the  ductility  of  steel. 

In  cast  iron  phosphorus  lengthens  the  period  of  solidi- 
fication of  the  cast  iron  in  the  mold,  and  makes  the  iron  more 
fluid.  With  a  high  phosphorus  content  (say  1.0  per  cent.) 
very  sharp  clean-cut  castings  are  produced,  but  they  are 
very  brittle.  Such  cast  iron  is  useful  for  castings  in  which 
brittleness  is  not  a  drawback,  and  where  sharpness  of 
form  is  of  great  importance,  e.g.,  in  castings  for  stoves  and 
kitchen  ranges,  decorative  work,  street  signs,  etc. 

Sulphur. — If  sulphur  combines  directly  with  iron  as 
ferrous  sulphide,  it  diminishes  the  strength  and  the  duc- 
tility of  iron.  If  manganese  is  present  the  sulphur  com- 
bines with  it  rather  than  with  the  iron  forming  manganese 
sulphide.  This  has  little  effect  on  the  strength  or  the  duc- 
tility of  the  iron,  but  by  many  metallurgists  it  is  claimed 
to  render  the  iron  liable  to  rapid  corrosion.  Sulphur  tends 
10 


146  MATERIALS  OF  ENGINEERING 

to  make  iron  brittle  when  red  hot — "red  short"  is  the 
term  used.  Sulphur  is  not -a  very  dangerous  ingredient 
in  rolled  steel  which  on  inspection  is  seen  to  be  free  from 
cracks,  because  the  presence  of  a  dangerous  amount  of 
sulphur  would  have  rendered  the  steel  so  "red  short" 
that  it  would  have  been  impossible  to  roll  it.  Sulphur 
is  an  ingredient  more  troublesome  to  the  steel  producer  than 
to  the  steel  user,  while  phosphorus  is  more  troublesome  to 
the  user  than  to  the  producer. 

The  maximum  allowable  sulphur  content  for  most  steels 
is  fixed  by  specification  at  0.05  per  cent,  though  some 
metallurgists  claim  that  a  sulphur  content  of  0.10  per  cent, 
is  not  necessarily  injurious  to  steel. 

In  cast  iron,  sulphur  tends  to  cause  the  carbon  of  the 
iron  to  assume  the  combined  form  rather  than  that  of 
graphite,  and  hence  tends  to  make  the  cast  iron  hard  and 
brittle. 

Manganese. — In  small  quantities  manganese  does  not 
have  a  great  direct  effect  on  the  properties  of  iron  and  steel, 
but  is  of  great  value  in  combining  with  any  sulphur  or 
oxygen  present,  and  thus  preventing  the  combination  of 
these  elements  with  iron.  Ordinary  steels  contain  from 
0.30  to  0.70  per  cent,  manganese.  A  manganese  content 
greater  than  about  7  per  cent,  makes  steel  very  strong  and 
tough,  but  so  hard  as  to  be  practically  unworkable.  Man- 
ganese steel  usually  contains  about  12  per  cent,  of  manga- 
nese, and  while  by  the  exercise  of  great  care  it  may  be 
forged  or  rolled,  it  is  usually  cast  directly  into  the  desired 
shape  of  the  finished  product.  It  is  used  for  the  jaws  of 
rock  crushers,  for  special  rails,  frogs,  and  crossings  for 
railroad  work,  for  burglar-proof  safes,  and,  in  general, 
where  extreme  hardness  or  resistance  to  wear  is  of  prime 
importance. 

Nickel. — A  nickel  content  of  about  3.5  per  cent,  makes 
steel  stronger  and  more  resistant  to  shocks.  Nickel  steel 
is  somewhat  more  expensive  than  ordinary  carbon  steel, 
but  is  widely  used  where  high  strength  is  necessary,  e.g., 
for  armor  plate,  automobile  axles,  gas-engine  valves,  special 


THE  EFFECT  OF  VARIOUS  INGREDIENTS       147 

rails,  long-span  steel  bridges.  Nickel  strengthens  steel 
without  reducing  its  ductility  to  any  great  extent,  hence 
nickel  steel  is  tough. 

Steel  alloyed  with  about  36  per  cent,  of  nickel  has  a 
very  low  coefficient  of  expansion  (about  one-sixth  that  of 
ordinary  iron) — lower  than  for  any  known  metal.  This 
steel  alloy  is  known  as  " invar  "  steel  and  is  used  in  making 
measuring  tapes  and  steel  scales,  and  in  measuring  instru- 
ments in  which  expansion  affects  the  accuracy  of  the  in- 
strument. Invar  steel  is  very  expensive  and  is  very  brittle, 
hence  invar  steel  measuring  tapes  can  not  stand  severe 
field  service. 

Chromium. — Chromium  makes  possible  a  steel  of  great 
hardness  and  strength.  Chromium  is  usually  used  in 
connection  with  nickel  or  vanadium  in  making  special 
grades  of  steel.  Chrome-nickel  steel  is  used  for  armor 
plate,  projectiles,  safes,  automobile  axles,  etc.  Chrome- 
nickel  steel  gears  are  in  extensive  use,  and  have  excellent 
wearing  qualities. 

An  iron  ore  mined  at  Mayari,  Cuba  contains  considerable 
quantities  of  nickel  and  of  chromium,  and  steel  made 
from  this  ore  is  a  natural  chrome-nickel  steel  of  high 
strength.  Mayari  steel  is  used  especially  for  making 
rails  and  track  bolts. 

Vanadium. — Vanadium  in  the  form  of  ferrovanadium 
adds  great  strength  to  steel,  and  seems  to  be  especially 
valuable  in  adding  resisting  power  against  repeated  applica- 
tions of  stress,  and  in  giving  steel  free  from  flaws  and  seams. 
Possibly  these  two  characteristics  are  inter-related.  Vana- 
dium steel  is  used  for  springs,  axles,  locomotive  frames,  and 
other  parts  of  railway  equipment;  it  is  also  used  in  auto- 
mobile construction.  Vanadium  seems  to  benefit  cast 
iron,  probably  on  account  of  its  tendency  to  remove 
oxygen  from  the  iron. 

Tungsten,  Molybdenum  and  Cobalt. — The  presence  of 
tungsten  and  molybdenum  in  steel  so  affects  the  critical 
temperature,  at  which  steel  changes  from  a  very  hard 
material  to  a  much  softer  material,  that  with  proper 


148  MATERIALS  OF  ENGINEERING 

heat-treatment  tungsten  and  molybdenum  steels  retain 
their  hardness  at  a  red  heat.  The  various  " high-speed" 
tool  steels  used  for  machine-tool  cutters  depend  for  their 
peculiar  properties  on  the  presence  of  tungsten  or 
molybdenum. 

Recently  cobalt  has  given  promise  of  usefulness  as  an 
alloying  element  for  high-speed  steel.  In  this  connection 
may  be  noted  the  high-speed  cutting  metal  Stellite,  which 
is  an  alloy  composed  mainly  of  cobalt,  chromium,  and 
molybdenum. 

Copper. — A  small  amount  of  copper,  not  more  than  1 
per  cent.,  has  no  marked  effect  on  the  strength  or  the 
ductility  of  steel,  but  is  claimed  by  some  metallurgists  to 
diminish  the  tendency  to  corrosion. 

Titanium. — In  making  pig  iron  the  presence  of  titanium 
in  the  ore  is  objectionable  as  it  tends  to  make  the  pig  iron 
" sticky7'  as  it  flows  from  the  blast  furnace.  Recently 
titanium  had  been  used  as  an  alloying  ingredient  for  steel, 
and  is  claimed  to  render  the  steel  more  uniform  in  quality 
throughout.  Titanium  has  been  used  in  making  rail  steel. 

The  Corrosion  of  Iron  and  Steel. — The  surface  of  iron 
or  steel  as  it  is  machined  is  a  silvery  white.  After  exposure 
to  the  air  the  surface  becomes  covered  with  a  thin  layer  of 
oxide.  With  prolonged  exposure  to  moist  air  this  film  of 
oxide  becomes  somewhat  deeper  and  the  surface  of  the  iron 
assumes  a  reddish-brown  color.  The  mere  formation  of  an 
evenly  distributed  coat  of  rust  (oxide)  does  very  little 
structural  injury  to  the  metal,  the  coat  of  oxide  soon  pro- 
tects the  iron  from  further  general  rusting.  There  is, 
however,  a  corrosive  action  known  as  " pitting'7  which 
sometimes  attacks  steel  and  iron  in  small  spots,  eating 
deep  holes  into  the  metal.  This  pitting  action  seems  to 
spread,  once  such  a  hole  is  started,  and  may  continue  until 
the  member  attacked  is  fatally  weakened. 

The  phenomena  of  corrosion  involve  very  complicated 
chemical  and  electrolytic  actions:  it  is  now  generally  ac- 
cepted that  destructive  corrosion,  or  pitting,  is  the  result 
of  electrolytic  action  between  the  pure  iron  and  the  im- 


THE  EFFECT  OF  VARIOUS  INGREDIENTS       149 

purities  mixed  with  it  in  steel,  and  manganese  sulphide  is 
suspected  of  being  an  active  corroding  agent.  It  is  claimed 
by  the  makers  of  wrought  iron  that  the  presence  of  slag  in 
wrought  iron  tends  to  inhibit  corrosion,  or  at  least  to  check 
its  spread  throughout  the  metal.  The  question  of  the 
relative  resisting  power  to  corrosion  of  wrought  iron  and  of 
steel  is  a  much-disputed  one.  Many  examples  are  given  of 
structures  of  both  materials  which  have  failed  by  corro- 
sion, and  many  structures  of  both  materials  have  given 
good  service  for  many  years.  Care  in  the  manufacture  of 
the  material  seems  to  be  of  about  as  much  importance  in 
producing  a  rust-resisting  material  as  does  the  nature  of 
the  process  used. 

Since  the  general  acceptance  of  the  electrolytic  theory  of 
corrosion  the  efforts  of  steel  manufacturers  to  produce  a 
rust-resisting  steel  have  been  directed  along  three  lines: 
(1)  to  produce  a  material  of  a  very  high  degree  of  purity 
so  that  they  will  be  present  very  few  foreign  ingredients 
to  set  up  electrolytic  action  with  iron;  (2)  to  put  into  the 
steel  some  ingredient  which  will  act  to  inhibit  electrolysis 
of  the  iron;  and  (3)  to  produce  a  specially  fine-grained  sur- 
face to  the  steel  by  working  it  in  special  rolls,  and  making 
it  dense  and  mechanically  resistant  to  corroding  action. 
All  these  methods  give  promise  of  usefulness,  but  none  has 
been  in  use  for  a  sufficient  period  of  time  to  demonstrate 
completely  its  value. 

Strength  and  Ductility  of  Iron  and  Steel. — In  Table  5 
are  given  average  values  for  the  proportional  limit,  ulti- 
mate strength,  and  ductility  of  the  common  grades  of  iron 
and  steel.  The  values  in  the  table  are  general  averages 
and  considerable  variation  from  them  may  be  expected 
in  individual  lots  of  metal. 

It  will  be  noted  in  Table  5  that  values  of  proportional 
limit  for  steel  in  compression  are  the  same  as  for  steel  in 
tension.  Recent  tests  by  the  U.  S.  Bureau  of  Standards 
indicate  that  for  thick  rolled  steel  the  proportional  limit 
and  the  yield  point  are  not  infrequently  found  to  be  lower 
for  compression  than  for  tension. 


150 


MATERIALS  OF  ENGINEERING 


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THE  EFFECT  OF  VARIOUS  INGREDIENTS       151 

Fig.  18  (page  39)  gives  typical  stress-strain  diagrams  up 
to  the  yield  point  for  the  common  grades  of  iron  and  steel. 

Selected  References  for  Further  Study 

STOUGHTON:  "The  Metallurgy  of  Iron  and  Steel,"  New  York,  1911,  Chaps. 

XI,  XV,  XVI. 
CAMPBELL:  "The  Manufacture  and  Properties  of  Iron  and  Steel,"   New 

York,  1907,  Chap.  XVII. 
HARBORD  AND  HALL:  "The  Metallurgy  of  Steel,"  London,   1916,  Vol.  I, 

Chaps.  XVI,  XVII. 

Topical  Discussion  of  the  Role  of  the  several  alloying  Elements  in  the 
Alloy  Steels,  Proceedings,   American    Society  for  Testing   Materials,   Vol. 
XVII,  Part  II,  p.  5  (1917). 
HIBBARD:  "Manufacture  and   Uses  of  Alloy  Steels,"  Bulletin   100,  U.  S. 

Bureau  of  Mines. 

Table  of  Mechanical  Properties  of  Materials,  U.  S.  Bureau  of  Standards, 
Letter  Circular  VII-1-1  (mimeograph  pamphlet)  partly  reprinted  in  Iron 
Age,  Aug.  28,  1919. 

References   on  the  Relative  Durability  of  Wrought  Iron  and   Steel 

HOWE  AND  STOUGHTON:  Relative  Corrosion  of  Iron  and  Steel,  Proceedings, 

American  Society  for  Testing  Materials,  Vol.  VIII,  p.  247  (1908). 
A.  M.  BYERS  COMPANY:  "An  Investigation  of  Pipe  Corrosion,"  Pittsburgh, 

Pa.     Bulletin  No.  30,  April,  1918.     A  trade  publication  presenting  the 

advantages  of  wrought  iron. 
CUSHMAN:  Corrosion  of  Steel  and  Its  Prevention,  Iron  Age,  May  23,  1912. 

Presents  the  advantages  of  a  very  low-carbon  steel  from  which  nearly 

all  impurities  have  been  removed. 
BUCK:  Copper  in  Steel — Its  Influence  on  Corrosion,  published  by  American 

Sheet  and  Tin  Plate  Co.,  Pittsburgh,  Pa.,  1913.     Presents  the  advan- 
tages of  steel  with  a  small  copper  content. 
NATIONAL  TUBE  Co.:  Modern  Welded  Pipe,  Nat'l  Tube  Co.,  Pittsburgh, 

Pa.     Presents  the  advantages  of  steel  which  has  been  given  thorough 

mechanical  working. 
AMERICAN  SOCIETY  FOR  TESTING   MATERIALS:  See  discussion  in  Annual 

Proceedings,  especially  for  1906,  1907,  1908,  1917, '1919. 


CHAPTER  XIV 
THE  NON-FERROUS  METALS  AND  ALLOYS 

Importance  of  Non-ferrous  Metals. — While  iron  and 
steel  are  by  far  the  most  important  of  the  metals  used  for 
structures  and  machines,  other  metals  are  by  no  means 
without  importance.  This  chapter  gives  a  very  brief 
treatment  of  the  occurrence  and  properties  of  some  of  the 
more  important  of  the  non-ferrous  metals,  and  of  some  of 
the  more  important  of  their  alloys. 

Copper. — In  the  United  States  thare  are  extensive  de- 
posits of  copper  ore  in  the  Lake  Superior  region  and  in  the 
Rocky  Mountain  region.  The  Lake  Superior  ores  contain 
native  copper  in  a  very  pure  form,  while  in  the  mines  of 
Montana,  Utah,  and  Arizona,  the  ore  is  in  the  form  of 
copper  sulphide  and  copper-iron  sulphide. 

Copper  is  obtained  from  sulphide  ores  by  a  complex 
process  involving  four  stages :  roasting,  smelting,  convert- 
ing (or  Bessemerizing)  and  electrolytic  refining.  Roasting 
of  copper  consists  in  partially  burning  out  the  sulphur  of 
the  sulphides  of  copper  and  iron  present  in  the  ores  leaving 
oxides  in  place  of  sulphides.  The  roasting  of  copper  ore 
is  carried  out  in  special  roasting  furnaces.  The  roasting 
stage  is  frequently  combined  with  the  smelting  stage. 

Smelting  of  the  roasted  ore  takes  place  either  in  a  blast 
furnace  or  in  a  " re verberatory  "  furnace.  In  the  smelting 
process  the  chief  change  is  the  removal  in  slag  of  the  earthy 
impurities,  and  the  production  of  a  mixture  of  metallic 
sulphides  of  iron  and  copper.  This  mass  is  known  as  matte. 

The  matte  is  further  purified  in  a  Bessemer  converter. 
The  converting  process  takes  place  in  two  stages.  In  the 
first  stage  the  air  blown  through  the  molten  matte  sets 
up  very  complicated  reactions,  the  result  being  the  oxida- 
tion of  the  iron  sulphide,  and  the  passing  of  the  iron  oxide 

152 


THE  NON-FERROUS  METALS  AND  ALLOYS     153 

formed  into  slag,  while  there  is  left  " white  metal"  which  is 
nearly  pure  copper  sulphide.  In  the  second  stage  this 
copper  sulphide  is  " blown7'  and  after  a  complicated  reac- 
tion crude  copper,  known  as  " blister"  copper  is  produced. 

In  the  electrolytic  refining  process  anodes  of  blister  cop- 
per are  placed  in  a  sulphuric  acid  bath.  The  sulphuric 
acid  is  obtained  from  the  products  of  roasting  the  sulphide 
ores.  A  cathode  of  very  pure  copper  is  used  and  from  the 
electrolytic  bath  very  pure  copper  is  produced.  "  Electro- 
lytic "  copper  is  the  purest  copper  on  the  market,  and  is  the 
only  kind  of  copper  used  for  electric  conductors. 

The  native  copper  (found  mainly  in  the  Lake  Superior 
region)  is  concentrated  from  its  ores,  though  if  copper  for 
electrical  conductors  is  to  be  produced  the  final  process  is 
that  of  electrolytic  refining. 

In  recent  years  deposits  of  low-grade  copper  ores  have 
been  successfully  worked  by  treating  the  ores  with  sulphuric 
acid,  and  then,  by  electrolysis  of  the  solution,  obtaining 
pure  copper. 

Uses  of  Copper. — Copper  is  widely  used  for  electrical 
wires,  on  account  of  its  high  electric  conductivity.  Next 
to  silver  it  is  the  best  conductor  known.  It  is  also  very 
extensively  used  as  a  constituent  of  alloys  with  tin,  zinc 
and  aluminium.  These  alloys  are  discussed  in  later  para- 
graphs of  this  chapter.  Copper  is  sometimes  used  for  the 
tubes  of  condensers  and  other  tubes  in  cases  where  the  cor- 
rodibility  of  iron  and  steel  render  them  unsuitable,  also  for 
small  tubes  which  must  withstand  high  pressure  and  yet 
be  flexible.  On  account  of  its  high  resistance  to  atmos- 
pheric corrosion,  copper,  in  the  form  of  rolled  plate,  is  used 
for  roofing  and  sheathing. 

Physical  Properties  of  Copper. — Copper  is  a  malleable 
and  ductile  metal  with  a  characteristic  reddish  color.  In 
Chap.  XI  was  discussed  the  variation  in  the  physical  prop- 
erties of  steel  due  to  mechanical  working.  Copper  exhibits 
even  greater  variability  in  physical  properties  due  to  treat- 
ment. For  cast  copper  the  ultimate  strength  is  about 
25,000  Ib.  per  square  inch.  It  has  no  yield  point.  A  poorly 


154  MATERIALS  OF  ENGINEERING 

defined  proportional  limit  is  found  at  about  8,000  Ib.  per 
square  inch.  It  has  an  elongation  in  8  in.  of  about  7  per 
cent.  When  cold-rolled  or  cold-drawn  (hard-drawn  is  a 
term  frequently  used),  copper  has  an  ultimate  of  from 
40,000  to  60,000  Ib.  per  square  inch  depending  on  the 
amount  of  reduction  by  drawing.  It  has  a  poorly  denned 
proportional  limit  of  about  three-quarters  the  ultimate,  and 
an  elongation  in  10  in.  of  about  3  per  cent.  Hard-drawn 
copper  may  be  softened  by  annealing,  and  its  physical 
properties  are  then  about  the  same  as  those  of  cast  copper. 
Data  on  the  compressive  strength  and  on  the  shearing 
strength  of  copper  are  very  few.  The  ultimate  in  compres- 
sion may  be  taken  as  0.75  the  ultimate  in  tension,  and  the 
ultimate  in  shear  as  0.6  the  ultimate  in  tension. 

Aluminum. — Aluminum  is  a  silvery  white  metal  of  con- 
siderable ductility  and  malleability  and  of  extreme  light- 
ness. All  clayey  earths  contain  a  high  percentage  of 
aluminum  usually  in  the  form  of  a  silicate,  but  sometimes 
in  the  form  of  an  oxide.  At  the  present  time  the  commer- 
cial ores  of  aluminum  contain  the  oxide  or  the  fluoride. 
The  pure  metal  is  produced  by  the  electrolysis  of  molten 
aluminum  oxide  which  is  protected  by  a  "slag"  of  alumi- 
num fluoride.  The  process  is  carried  on  in  an  electric  fur- 
nace and  the  electric  current  furnishes  heat,  and  also  causes 
the  separation  of  the  metal  from  its  ore. 

Uses  of  Aluminum. — Aluminum  is  used  in  metallurgical 
processes  on  account  of  its  property  of  " quieting"  molten 
metals  which  without  its  addition  would  boil  vigorously 
under  the  action  of  escaping  gas. 

Aluminum  is  used  alloyed  with  small  amounts  of  tin  and 
of  zinc,  and  with  larger  amounts  of  copper  to  form  several 
valuable  alloys  which  are  discussed  later  in  the  chapter. 
Aluminum  either  cast  or  rolled  is  used  for  the  material  of 
machine  parts  in  which  weight  is  objectionable  and  in 
which  great  strength  is  not  necessary;  for  example,  for  the 
crank  and  gear  cases  of  motor  cars.  It  is  also  used  for 
electrical  conductors  on  account  of  its  high  conductivity 
(about  60  per  cent,  that  of  copper),  light  weight,  and 


THE  NON-FERROUS  METALS  AND  ALLOYS     155 

(in  the  form  of  hard-drawn  wire)  fairly  high  strength. 
The  burden  put  on  the  supporting  towers  and  poles  of  an 
electrical  power  transmission  line  by  the  dead  weight  of 
wire  is  about  half  as  much  if  the  conductors  are  of  aluminum 
as  when  they  are  of  copper.  jOn  account  of  its  high  resist- 
ance to  corrosion  aluminum  is  extensively  used  for  cooking 
utensils. 

Properties  of  Aluminum. — The  salient  property  of  alu- 
minum is  its  extreme  lightness.  It  weighs  about  0.093  Ib. 
per  cubic  inch  as  compared  with  0.280  Ib.  per  cubic  inch  for 
steel.  Cast  aluminum  has  an  ultimate  strength  of  about 
13,000  Ib.-per  square  inch  with  a  poorly  defined  propor- 
tional limit  of  about  9000  Ib.  per  square  inch.  Hard- 
drawn  aluminum  wire  has  an  ultimate  of  about  30,000  Ib. 
per  square  inch  and  a  poorly  defined  proportional  limit  of 
about  20,000  Ib.  per  square  inch.  The  ultimate  in  com- 
pression may  be  taken  as  0.75  the  ultimate  in  tension,  and 
the  ultimate  in  shear  as  0.6  the  ultimate  in  tension.  Alu- 
minum resists  very  strongly  the  corroding  influence  of  the 
atmosphere,  but  is  readily  attacked  by  strong  alkalies  or 
organic  acids. 

Zinc. — Zinc  is  a  bluish-white  metal  found  in  nature 
mainly  in  the  form  of  the  carbonate  or  the  sulphide.  In 
the  United  States  there  are  extensive  deposits  of  zinc  ore  in 
New  Jersey,  in  southwest  Missouri,  and  in  southwest  Wis- 
consin. Zinc  ore  is  first  roasted  to  transform  the  sulphide 
or  the  carbonate  to  zinc  oxide.  The  oxide  is  then  heated 
with  charcoal  or  some  other  cheap  form  of  carbon.  The 
carbon  combines  with  the  oxygen  of  the  oxide;  pure 
zinc  is  separated  from  the  ore,  is  volatilized,  and  the  metal 
vapor  is  conducted  to  cooling  chambers  where  it  is  con- 
densed to  liquid  form  and  poured  into  small  ingots.  Zinc 
thus  produced  is  commonly  known  as  spelter.  Spelter  is 
remelted  and  rolled  into  sheets.  The  rolling  is  done  at  a 
temperature  of  about  250°F.,  and  it  is  important  that  the 
temperature  be  maintained  very  closely  during  rolling, 
as  zinc  is  much  less  ductile  at  temperatures  either  higher 
or  lower. 


156  MATERIALS  OF  ENGINEERING 

Properties  of  Zinc. — Zinc  weighs  about  0.27  Ib.  per 
cubic  inch.  Cast  zinc  has  an  ultimate  strength  of  about 
9,000  Ib.  per  square  inch,  but  no  well-defined  proportional 
limit.  Rolled  zinc  has  an  ultimate  of  about  24,000  Ib.  per 
square  inch  and  a  poorly  defined  proportional  limit  of  about 
5,000  Ib.  per  square  inch.  Zinc  resists  atmospheric  corro- 
sion strongly,  but  is  readily  attacked  by  acids.  Of  all  the 
commercial  metals  zinc  is  the  most  electronegative.  Zinc 
properly  rolled  is  highly  ductile. 

Uses  of  Zinc. — The  electronegative  action  of  zinc  makes 
it  useful  in  electric  batteries,  it  is  the  negative  element  in 
practically  all  primary  batteries.  Zinc  is  widely  used  as 
a  protective  coating  against  corrosion  on  iron  and  steel 
plates.  Plates  so  protected  are  known  as  galvanized  iron, 
when  the  coating  is  applied  by  dipping  the  plate  in  molten 
zinc,  or  by  plating  the  zinc  on  the  iron  by  electrolysis;  and 
are  known  as  sherardized  iron  when  the  coating  is  formed 
by  the  condensation  of  volatilized  zinc  dust  on  the  iron. 
To  a  limited  extent  zinc  plates  are  used  for  roofing  and 
spouting  for  houses  on  account  of  the  corrosion-resisting 
powers  of  zinc.  Zinc  is  rarely  used  as  a  stress-carrying 
member  of  a  machine  or  structure ;  in  some  cases  zinc  strips 
have  been  used  to  support  electric  cables  which  are  exposed 
to  the  corroding  action  of  the  atmosphere.  Zinc  is  an 
important  ingredient  in  various  alloys,  and  its  use  as  an 
alloy  material  is  discussed  in  succeeding  paragraphs. 

Non-ferrous  Alloys. — A  large  variety  of  useful  metals 
are  produced  by  melting  together  various  combinations  of 
the  commercial  non-ferrous  metals.  The  study  of  the  na- 
ture of  the  alloys  thus  produced  is  an  interesting  chemical 
problem.  In  some  cases  the  resulting  alloy  seems  to  be 
merely  a  mixture  of  the  elementary  metals  which  may  be 
present  in  almost  any  proportion;  in  other  cases  the  alloy- 
ing of  certain  definite  proportions  of  metals  seem  to  pro- 
duce definite  chemical  compounds.  In  many  cases  the 
properties  of  the  alloy  differ  widely  from  those  of  the  con- 
stituent metals.  The  properties  of  an  alloy  depend  not 
merely  on  its  chemical  composition,  but  also  on  the  methods 


THE  NON-FERROUS  METALS  AND  ALLOYS     157 


used  in  producing  it,  and  on  the  mechanical  treatment  it 
receives. 

Copper-zinc  Alloys;  Brasses. — Copper  and  zinc  alloyed 
produce  brass,  one  of  the  commonest  of  alloys.  Copper 
and  zinc  can  be  alloyed  in  any  proportion.  The  average 
physical  properties  of  brass  with  varying  percentages  of 
copper  and  zinc  are  shown  in  Fig.  64,  which  is  self-explana- 


I 

-,'Dafa  lacking  for  frits 
Curve. 


10 


FIG. 


20     30     40      50     60      70 
Percentage   of    Copper 
64.  —  Properties  of  copper-zinc  alloys  (brasses). 


80      90     100 


The  modulus  of  elas- 


ticity for  brass  varies  from  9,000,000  to  14,000,000  Ib.  per  sq.  in.,  averaging  about 
13,000,000  Ib.  per  sq.  in. 

tory.  Brass  can  be  cast  directly  into  shape,  and  can  be 
rolled  or  drawn  into  sheets,  tubes,  rod,  and  wire.  It 
corrodes  less  easily  than  iron  or  steel,  and  finds  a  wide  use 
for  hydraulic  fittings,  pump  linifigs,  and  in  places  where 
prolonged  exposure  to  moisture  is  necessary.  Brass  costs 
seven  or  eight  times  as  much  as  iron  or  steel.  Brass  is  a 
useful  metal  for  bearings.  If  it  is  attempted  to  run  a  steel 


158 


MATERIALS  OF  ENGINEERING 


shaft  in  a  steel  bearing,  the  rubbing  surfaces  cut  and  tear 
each  other.  If,  however,  a  softer  metal,  such  as  brass,  is 
used  as  a  bearing  metal  against  steel,  a  smooth  surface  is 
worn,  and  cutting  is  not  so  likely  to  take  place. 

Copper-tin  Alloys;  Bronzes.— Copper  and  tin  like  copper 
and  zinc  can  be  alloyed  in  any  proportion.  The  proper- 
ties of  bronze,  as  the  alloy  of  copper  and  tin  is  called,  for 


cr 

™  100,000 

Si. 


£    50,000 


10     20 


90 


30     40      50     60      70      80 
Percentage   of    Copper 

FIG.  65. — Properties  of  copper-tin  alloys  (bronzes).  The  modulus  of  elasticity 
for  bronze  varies  from  9,000,000  to  17,000,000  Ib.  per  sq.  in.,  averaging  about 
15,500,000  Ib.  per  sq.  in. 

varying  proportions  of  the  ingredients  are  shown  in  Fig.  65, 
which  is  self-explanatory.  Bronze  can  be  cast  into  shape 
or  rolled  into  wire,  rods,  and  sheets.  Its  resists  corrosion 
even  better  than  brass,  it  is  more  expensive  than  brass, 
and  its  uses_are,  in  general,  the  same.  The  terms  "brass" 


THE  NON-FERROUS  METALS  AND  ALLOYS     159 

and  " bronze"  are  somewhat  loosely  used  in  practice,  either 
term  being  frequently  used  to  denote  any  yellow  metal 
containing  copper  in  large  proportion. 

"  Season  "  and  Corrosion  Cracking  of  Brass  and  Bronze. 
—Brass  and  bronze  rods,  tubes,  and  sheets  which  are  appar- 
ently made  of  perfectly  sound  metal  occasionally  develop 
cracks  under  light  service,  or  while  in  storage  under  no 
external  load.  This  phenomenon  is  rather  inaccurately 
known  as  " season"  cracking.  It  is  accelerated  by  the 
presence  of  moisture  or  moist  air,  which  tends  to  corrode 
the  metal.  Hence  the  phenomenon  is  also  known  as  corro- 
sion cracking.  This  cracking  may  also  be  caused  by  sud- 
den changes  of  temperature.  Brass  with  a  composition 
varying  between  20  per  cent,  zinc  and  45  per  cent,  zinc 
seems  the  metal  most  liable  to  season  cracking,  but  other 
brasses  and  bronzes,  and  other  copper  alloys  also  suffer 
from  this  defect.  Season  cracking  is  confined  to  brass 
which  has  been  cold-worked,  and  is  caused  primarily  by 
the  initial  stresses  set  up  by  that  cold-working.  Corrosion 
by  roughening  the  surface  sets  up  localized  stresses  at  the 
root  of  the  minute  pits  and  corrugations  formed,  and  thus 
is  a  contributary  cause  toward  cracking. 

Season  cracking  may  be  prevented  by  removing  the 
initial  stress,  this  can  sometimes  be  done  by  annealing,  but 
in  many  cases  annealing  leaves  the  brass  too  weak,  especially 
in  the  case  of  brass  for  springs.  Very  careful  heat  treat- 
ment may  reduce  the  initial  stresses  to  a  low  value  without 
reducing  the  strength  greatly,  but  the  manipulation  is 
rather  too  delicate  for  shop  conditions.  The  introduction 
of  neutralizing  stresses  by  " springing"  the  metal  in  the 
proper  direction  is  used  with  some  success.  Keeping  the 
surface  of  brass  parts  polished  prevents  localized  stress 
at  the  root  of  scratches  and  minute  corrosion  pits,  and 
tends  to  inhibit  season  cracking. 

Three -metal  Alloys. — Alloys  with  very  high  strength  and 
ductility  can  be  produced  by  the  proper  mixture  of  copper, 
zinc,  and  tin.  Table  6  gives  the  approximate  composi- 
tion and  the  physical  properties  of  a  number  of  common 


160 


MATERIALS  OF  ENGINEERING 


TABLE  6. — AVERAGE  VALUES  FOR  STRENGTH  AND  DUCTILITY  OF  VARIOUS 
METALS  AND  ALLOYS 

Values  for  the  strength  and  ductility  of  brass  (copper-zinc  alloys)  and 
bronze  (copper-tin  alloys)  given  in  Fig.  63  and  Fig.  64.  Values  given  are 
based  on  test  data  from  various  materials  testing  laboratories.  Values  for 
the  strength  of  pure  copper,  aluminum,  and  zinc  are  given  in  the  text  of 
Chap.  XIV. 


Metal  or  alloy 

Approximate  composition, 
per  cent. 

Weight, 
Ib.  per 
cu.  in. 

Strength  in 
tension,0 
Ib.  per  sq.  in. 

Elonga- 
tion in 
2  in., 
per 

cent. 

Propor- 
tional 
limit 

Ultimate 
tensile 
strength 

Aluminum  bronze: 
Cast 

J  Copper  90,  aluminum  [ 

i"            1 

}  Copper    65,    zinc    30,  f 
J  iron  5                                 \ 

\  Copper    58,    zinc    40,  ( 

/  tin  2                                   \ 

1  Copper    GO,    zinc    35,  f 
J  lead  5                               \ 

f  Copper  83,  tin  4,  lead  \ 
\  6,  zinc  7                           J 

f  Copper    88,     tin     10,  \ 
\  lead  2                               J 

(  Copper    60,    zinc    39,  ) 
j  traces  of  iron  and  of  ^ 
(  manganese                      J 

(Copper    95,    tin    4.9,   1 
phosphorus,  trace          | 
J 

f  Copper    88,     tin     10,  1 
\  zinc  2                                / 

(  Nickel  67,  copper  28,  1 
\  iron  +  carbon  + 
[  manganese  -(-silicon,  5  j 

J  Lead  100 
}  Tin  100                            / 

Lead  90,  antimony  10 

0.27 

0.29 
0.31 
0.31 
0.32 
0.30 

0.32 

0.31 
0.32 

0.41 
0.26 

25,000 
30,000 
80,000 

| 

60,000 
70,000 
90,000 

45,000 
65,000 

60,000 
79,000 

35,000 
60,000 

30,000* 
32,000 

70,000' 
75,000 

32,000 
65,000 

105,000 
35,000 

72,000* 
85,000 

1,700" 
3.300/ 

4,000" 
5,300* 

6,900 

25 
30 
10 

10 

17 

35 

28 
30 

17 

7 

i     '.   ' 

25 
25 

7 
30 

5 
17 

34 

42 

35 

Rolled  

Cold-drawn  
Delta  metal: 
Cast 

Rolled  

25,000 
54,000 

16,000 
18,000 

30,000 
45,000 

16,000 
40,000 

Tobin  bronze: 
Cast  

Rolled  
Yellow  brass: 
Cast  

Rolled  

Red  brass: 
Cast  

Soft  gear  bronze: 
Cast  

Manganese  bronze: 
Cast  
Rolled  

Phosphor  bronze: 
Cast  

Rolled 

Hard-drawn      spring 
wire  
Admiralty  gun  metal: 
Cast  

10,000 

37,000 
50,000 

Monel  metal 
Cast 

Rolled 

Lead: 
Cast  

Rolled  
Tin: 
Cast  
Rolled..'  

1,600 
4,000 

Antimony-lead  : 
Cast 

THE  NON-FERROUS  METALS  AND  ALLOYS     161 

three-metal  alloys.  It  should  be  remembered  that  the 
properties  of  an  alloy  depend  largely  on  the  proper  foundry 
treatment  of  the  ingredients  while  the  alloy  is  being  melted, 
such  as  temperature  of  pouring,  and  on  the  mechanical 
treatment  after  melting,  such  as  rolling  or  drawing  out, 
as  well  as  on  the  chemical  composition.  The  properties 
given  in  Table  6  would  be  found  only  in  alloys  manufac- 
tured under  good  foundry  conditions. 

The  effect  of  cold-rolling  and  cold-drawing  on  the  proper- 
ties of  alloys  is  similar  to  the  effect  on  iron  and  steel, 
namely  a  raising  of  the  elastic  strength  of  the  material, 
and,  especially  in  the  case  of  the  copper  alloys,  a  raising 
of  the  ultimate  also. 

Alloys  of  Aluminum. — Aluminum  is  alloyed  with  copper, 
magnesium,  zinc  and  other  metals.  Light  aluminum  alloys 
are  composed  mainly  of  aluminum,  which  gives  lightness ; 
the  alloying  metals  give  greater  strength  than  that  of  pure 
aluminum.  Like  brasses  and  bronzes  aluminum  alloys  are 
made  stronger  by  cold-drawing. 

Aluminum  is  used  as  an  alloying  element  in  alloys  in 
which  the  principal  metal  is  copper,  zinc,  tin,  or  a  combina- 
tion of  two  or  more  of  these  metals. 

Table  6  gives  values  of  the  mechanical  properties  of  a 
number  of  the  alloys  with  small  aluminum  content.  Table 
7  gives  values  of  the  mechanical  properties  of  a  number  of 
light  aluminum  alloys. 

Special  Alloys. — Space  is  lacking  to  enumerate  all  the 
alloys  of  metals  in  common  use.  A  few  alloys  of  special 
significance  will  be  briefly  noted;  the  mechanical  properties 
of  a  somewhat  larger  number  of  alloys  are  given  in  Table  6. 

NOTES  TO  TABLE  6 

0  If  special  values  for  compressive  strength  are  not  given  the  ultimate  in  compression 
may  be  safely  taken  as  equal  to  the  strength  at  the  proportional  limit  in  tension.  Strength 
in  shear  may  be  taken  as  0.6  of  the  strength  in  tension. 

6  Ultimate  in  compression,  77,000  Ib.  per  sq.  in. 

J  Modulus  of  elasticity  in  tension,  14,000,000,  Ib.  per  sq.  in. 

*  Modulus  of  elasticity  in  tension,  22,000,000  Ib.  per  sq.  in. 

*  Modulus  of  elasticity  in  tension,  700,000  Ib.  per  sq.  in. 

/  Modulus  of  elasticity  in  tension,  1,000,000  Ib.  per  sq.  in. 
a  Ultimate  in  compression,  6400  Ib.  per  sq.  in. 

*  Modulus  of  elasticity  in  tension,  4,000,000  Ib.  per  sq.  in. 

11 


162 


MATERIALS  OF  ENGINEERING 


TABLE  7. — AVERAGE  VALUES  FOR  STRENGTH  AND  DUCTILITY  OF  ALUMINUM 

AND  ALUMINUM  ALLOYS 

The  values  given  in  this  table  are  based  on  data  from  various  testing 
laboratories. 


Metal 

Weight 
Ib.  per 
cu.  in. 

Tensile  strength,0 
Ib.  per  sq.  in. 

Elonga- 
tion in 
2  in. 
per 
cent. 

Propor- 
tional 
limit 

Ulti- 
mate 

Commercial  aluminum,  99  per  cent,  pure: 
Cast                                                                

0.093 
0.097 
0.097 
0.097 

0.104 
0.104 

0.104 

0.102 
0  102 

9,000 
8,500 
20,000 
30,000 

11,500 
35,000 

13,000 

13,000 
13,500 
30,000 
40,000 

19.500 
41,000 

19,000 

35,000 
56,000 
55,000 

20,300 
38,200 

22,000 

21,000 
22,300 
27,900 

35,000 

20 
23 
4 
6 

12 
5 

2 

22 
2 
15 

5 

10 

7 

9 
22 

8 

2 

Rolled  and  annealed                      

Wire,  hard  drawn  
Aluminum  96  per  cent.,  copper  4  per  cent.: 
Cast         

Hard  rolled                              .             

Aluminum  92  per  cent.,  copper  8  per  cent.: 
Cast                                                                

"Duralumin,"  aluminum  96  per  cent.,  magnesium 
1.5  per  cent.,  copper  2.5  per  cent.,  iron  and 
silicon,  trace: 

0.102 

0.102 
0.102 

0.091 

0.093 
0.093 
0.093 

0.106 

14,300 
27,100 

8,000 

9,000 
13,500 
22,900 

14,000 

Aluminum  96  per  cent.,  copper  2  per  cent.,  mang- 
anese 2  per  cent.: 
Cast             

Rolled         

"Magnalium,"  aluminum  95  per  cent.,  magnesium 
5  per  cent.: 
Cast            

Aluminum  95  per  cent.,  nickel  5  per  cent.: 
Cast            

Rolled         

Aluminum  81  per  cent.,  copper  3  per  cent.,  zinc  16 
per  cent.: 
Cast                

a  The  compressive  strength  of  the  metals  in  this  table  may  safely  be  taken  at  values  equal 
to  the  corresponding  proportional  limit  in  tension.  The  shearing  strength  may  safely 
be  taken  at  0.6  of  the  tensile  strength. 

Aluminum  bronze  is  an  alloy  of  aluminum  with  copper. 
The  aluminum  gives  the  alloy  lightness,,  while  the  addition 
of  copper  to  pure  aluminum  increases  its  strength. 

Manganese  bronze  is  an  alloy  of  copper  with  manganese 
and  a  little  iron.  The  manganese  by  its  strong  affinity 
for  oxygen  " cleanses"  the  metal  of  any  small  particles  of 
oxide.  Manganese  bronze  has  a  very  high  strength,  about 
equal  to  that  of  strucural  steel.  Manganese  bronze  resists 


THE  NON-FERROUS  METALS  AND  ALLOYS     163 

corrosion  by  either  salt  or  fresh  water  remarkably  well 
and  is  used  for  propeller  wheels  and  other  parts  of  ships. 

Phosphor  bronze  is*  prepared  by  the  addition  of  a  little 
phosphorus  to  a  copper-tin  alloy.  The  phosphorus  itself 
has  but  little  effect  on  the  physical  properties  of  the  bronze 
but  it  unites  with  any  oxide  present,  and  " cleanses"  the 
alloy  from  the  injurious  effects  of  the  oxide. 

Monel  metal  is  an  alloy  of  about  67  per  cent,  nickel,  28 
per  cent,  copper,  and  the  remaining  5  per  cent,  comprising 
iron,  manganese,  silicon,  and  carbon.  It  has  a  strength 
equal  of  that  of  mild  steel,  and  a  high  degree  of  ductility. 
It  resists  corrosion  to  remarkable  degree,  and  also  resists  the 
action  of  dilute  acids,  saline  solutions,  and  many  alkaline 
liquids,  unless  conditions  bring  about  electrolytic  action. 
Monel  metal  retains  its  strength  at  high  temperatures  to  a 
greater  degree  than  any  other  commercial  metal  with  the 
possible  exception  of  cold-drawn  steel.  It  has  been  used 
for  steamship  propellers,  roofing  for  large  railway  terminals, 
steam  turbine  blades,  and  valves  subjected  to  high  tempera- 
ture, pump  rods  and  pistons  for  pumps  designed  to  handle 
corrosive  liquids. 

Bearing  Metals. — In  choosing  metals  for  bearing  sur- 
faces where  shafts  turn  in  bearings  or  members  slide  on  one 
another  it  is  of  prime  importance  that  the  bearing  metals 
should  have  sufficient  compressive  strength  to  carry  the 
bearing  pressure,  should  wear  to  smooth  surfaces  as  they 
rub  together,  and  should  develop  a  minimum  of  friction 
when  they  actually  come  in  contact,  as,  for  example,  when 
a  shaft  is  starting  or  stopping.  All  satisfactory  bearing 
metals  have  a  characteristic  crystalline  structure  made  up 
of  hard  crystals  alternating  with  relatively  soft  crystals. 
The  hard  crystals  support  the  load  and  resist  wear;  the  softer 
crystals  suffer  some  plastic  deformation  and  permit  the  hard 
crystals  to  adjust  themselves  to  the  surface  requirements  of 
the  rotating  shaft  or  the  sliding  member.  Moreover,  the 
softer  crystals  wear  slightly  below  the  surface  of  the -harder 
crystals  and  thus  form  minute  depressions  on  the  surface 
which  retain  lubricant.  This  retained  lubricant  is  sufficient 


164  MATERIALS  OF  ENGINEERING 

to  keep  the  bearing  from  undue  friction  and  heating  when 
starting  or  stopping.  Around  a  properly  lubricated  shaft 
in  motion  is  a  film  of  lubricant,  so*  that  the  nature  of  the 
bearing  metal  is  not  of  much  consequence  once  the  shaft 
is  'up  to  speed,  but  the  lubricant-retaining  nature  of  good 
bearing  metal  is  of  great  consequence  during  starting  and 
stopping.  Usually  it  is  found  advisable  to  make  the  shaft 
or  other  moving  part  of  hard  metal  and  the  bearing  face 
of  soft  metal.  Steel  on  cast  iron,  steel  on  brass  or  bronze, 
and  steel  on  various  special  soft  " bearing  metals"  are 
examples  of  good  wearing  surfaces.  The  faces  of  bearings 
are  frequently  made  of  special  " bearing  metals"  which  are 
soft  and  which  melt  at  a  low  temperature.  Bearing  facings 
made  of  such  metals  can  be  cast  directly  in  place,  and  usu- 
ally require  no  machining. 

The  best  known  of  such  bearing  metals  is ' '  Babbitt  metal, ' ' 
which  has  the  approximate  composition  tin  89  parts,  copper 
4  parts,  antimony  7  parts.  Another  bearing  metal  has  the 
approximate  composition,  lead  80  per  cent.,  antimony  20 
per  cent.  Lead  alone  is  too  soft  for  a  bearing  metal,  and 
the  addition  of  antimony  is  necessary  to  give  the  requisite 
hardness.  A  large  number  of  special  bearing  metals  are  on 
the  market,  most  of  them  alloys  of  lead,  tin,  and  antimony. 

Selected  References  for  Further  Study 

HOFMAN:  "The  Metallurgy  of  Copper/'  New  York,  1916.  A  recent  com- 
prehensive treatise  by  an  American  metallurgist. 

GILLETT:  Brass  Furnace  Practice  in  the  United  States,  U.  S.  Bureau  of 
Mines,  Bulletin  73. 

WEBSTER:  Considerations  Affecting  Specifications  for  Wrought  Non- Fer- 
rous Materials,  Proceedings  of  the  American  Society  for  Testing  Mate- 
rials, Vol.  XIV,  Part  II,  p.  128  (1914). 

TROOD  :  Theory  and  Practice  of  Sherardizing,  Iron  Age,  July  23,  30,  Aug.  6, 
13,  20,  1914. 

CORSE  AND  COMSTOCK:  Aluminum  Bronze.  Some  Recent  Tests,  Proceed- 
ings of  the  American  Society  for  Testing  Materials,  Vol.  XVI,  Part  II. 
p.  117  (1916). 

"Aluminum  and  Its  Light  Alloys,"  Circular  No.  76,  U.  S.  Bureau  of  Stand- 
ards, Washington,  D.  C.  1919. 

"  Topical  Discussion  on  Season  and  Corrosion  Cracking  of  Brass,"  Proceed- 
ings, American  Society  for  Testing  Materials,  Vol.  XVIII,  part  II,  p. 
147  (1918). 

LYNCH:  A  Study  of  Bearing  Metals,  Proceedings  of  the  American  Society 
for  Testing  Materials,  Vol.  XIII,  p.  699  (1913). 


CHAPTER  XV 
TIMBER 

Uses  in  Engineering  Construction. — Timber  has  been 
used  as  a  structural  material  since  the  earliest  tinier.  It 
is  less  costly  than  iron,  steel,  concrete,  or  brick;  it  is  light 
and  handled  with  ease,  and  it  can  be  readily  sawed  and  cut 
to  almost  any  desired  shape.  On  the  other  hand  it  is  easily 
destroyed  by  fire  and  is  subject  to  decay  and  these  facts 
greatly  diminish  its  value  for  permanent  construction. 
In  general,  at  the  present  day  timber  is  used  for  cheap  or 
for  temporary  construction,  for  structural  members  (nota- 
bly railway  ties)  which  must  possess  a  high  degree  of  re- 
silience under  shock,  and  for  small  members  for  which 
lightness  is  especially  important. 

Principal  Varieties  of  Structural  Timber. — The  com- 
mercial varieties  of  wood  are  divided  into  two  general 
classes,  soft  wood  and  hard  wood.  The  distinction  between 
hard  wood  and  soft  wood  is  not  entirely  logical.  Some  of 
the  harder  and  stronger  of  the  "soft"  woods  (e.g.,  yellow 
pine)  surpass  in  hardness  and  strength  the  softer  species  of 
"hard"  wood  (e.g.,  poplar).  The  term  soft  wood  is  applied 
to  wood  from  any  one  of  the  numerous  cone-bearing  trees 
of  which  the  pines,  the  spruces,  the  hemlock,  the  fir  the 
tamarack,  the  cedar,  the  cypress  aud  the  redwood  are  the 
principal  species.  Nearly  all  cone-bearing  trees  are  "ever- 
greens." The  term  hard  wood  is  applied  to  wood  from  the 
"broad-leaved"  trees  some  of  the  commonest  of  which  are: 
the  white  oak,  the  red  oak,  the  ash,  the  hickory,  the  poplar, 
the  maple,  the  walnut,  the  chestnut,  the  breech,  the  catalpa, 
the  eucalyptus,  and  the  mahogany.  For  general  struc- 
tural purposes  the  soft  woods  are  much  more  generally 
used  than  are  the  hard  woods.  The  principal  uses  of  the 
hard  woods  are  for  interior  finish,  furniture,  cabinet  work, 
and  the  like,  though  white  oak  and  red  oak  are  used  for 

165 


166 


MATERIALS  OF  ENGINEERING 


railroad  ties,  and  hickory  and  ash  are  used  for  the  wooden 
parts  of  vehicles  and  agricultural  implements.  Table  8 
gives  the  characteristics  and  the  uses  of  the  more  common 
kinds  of  wood. 

TABLE  8. — CHARACTERISTICS  AND  USES  OF  WOOD 

See  Fig.  66  for  location  of  regions  in  the  United  States  producing  'dif- 
ferent species  of  wood. 


Species 


Characteristics 


Uses 


Soft  woods : 

Yellow  pine 


White  pine 

Norway  (red)  pine.. 
Western  pine 


Douglas  fir  (Oregon 
fir) 

Hemlock 

Tamarack  (larch) 

Spruce 

Cedar. 

Redwood 


Cypress. 


Hard  woods : 
White  oak. 


Red  oak.. 
Hickory... 

Maple.  .  .  . 


Ash 

Elm.. 


Heavy,  hard,  strong,  tough, 
coarse  grained,  decays  in  con- 
tact with  soil. 

Light,  soft,  straight  grained, 
not  very  strong. 

Light,  hard,  corase  grained. 

Trade  name  for  a  number  of 
kinds  of  wood,  general  char- 
acteristics like  somewhat  like 
Norway  pine. 

Hard,  strong,  wide  variations  in 
quality,  durable,  rather  diffi- 
cult to  work. 

Soft,  light,  brittle,  splits  easily, 
not  durable. 

Hard,  heavy,  strong,  durable. 

Light,  soft,  close  grained, 
straight  grained,  resists  decay 
and  marine  boring  insects. 

Soft,  light  fine  grained,  very 
durable. 

Light,  soft,  weak,  brittle,  coarse 
grained,  straight  grained,  easy 
to  work,  durable  in  contact 
with  soil. 

Very  durable,  light,  hard,  close 
grained,  easily  worked,  takes 
high  polish. 

Heavy,  strong,  tough,  close 
grained,  splits  with  difficulty, 
checks  if  not  carefully  seas- 
oned, takes  high  polish. 

Softer,  weaker,  and  more  porous 
than  white  oak. 

Heaviest,  hardest  and  toughest 
of  American  woods.  Attacked 
by  boring  insects. 

Heavy,  hard,  strong,  coarse 
grained.  Takes  good  polish. 

Heavy,  hard,  birttle,  "springy." 

Heavy,  hard,  strong,  tough, 
very  close  grained,  difficult  to 
split  and  shape,  warps  badly. 


Heavy  framing  timbers,  flooring. 


Pattern  making,  interior  finish. 

All  kinds  of  construction. 
General  construction  work. 


All  kinds  of  construction. 


Cheap  framing  timber,  boxes  and 
crates. 

Posts,  poles,  ship  timbers,  ties 
sills. 

Framing  timbers,  piles,  under- 
water construction. 

Water  tanks,  shingles,  posts, 
fencing,  boat  building. 

Ties,  posts,  poles,  general  con- 
struction work. 


House    siding,    shingles,    poles, 
building  lumber,  interior  finish. 


Framed  structures,  interior  fin- 
ish, ties,  vehicle  and  furniture 
making. 

Ties,  furniture,  interior  finish. 

Vehicles,  handles,  agricultural 
implement  manufacture. 

Flooring,  interior  finish,  furni- 
ture. 

Interior  finish  and  cabinet  work. 
Vehicle  and  ship  building,  sills. 


TIMBER  167 

Production  of  Timber  in  the  United  States.— Fig.  66  is 
a  map  of  the  United  States  showing  the  location  of  the  prin- 
cipal supplies  of  timber.  The  future  supply  of  timber  is 
a  question  of  great  importance  to  structural  engineers. 
At  the  present  time  the  annual  consumption  of  timber  in 
the  United  States  is  about  four  times  the  amount  of  the 
annual  growth  of  timber.  Already  several  regions  for- 
merly supplying  great  quantities  of  timber  are  practically 
deforested,  and  the  price  of  timber  shows  a  marked  rise  from 
decade  to  decade.  It  seems  probable  that  in  the  future 
many  structures  now  built  of  timber  will  be  built  of  steel, 
concrete,  or  brick.  Efforts  to  insure  an  adequate  supply  of 
timber  give  some  promise  of  success  along  several  lines 
among  which  are:  the  systematic  growing  of  timber  on 
land  not  suitable  for  food  crops,  economy  in  the  use  of 
wood  through  a  careful  study  of  the  physical  properties 
of  various  species  and  the  utilization  of  species  formerly 
thought  unavailable,  and  such  methods  of  lumbering  as 
will  minimize  the  danger  of  great  forest  fires,  which  in  past 
years  in  this  country  have  destroyed  as  much  timber  as  has 
been'used  for  all  structural  purposes. 

The^first  step  in  the  production  of  commercial  lumber 
is  "logging,"  that  is  the  felling  of  trees  in  the  forest,  trim- 
ming off  branches  and  vegetation,  cutting  the  trunks  and 
limbs  to  sizes  which  can  be  handled,  and  transporting  the 
resulting  logs  to  the  saw  mill.  Methods  of  felling  trees, 
and  of  transporting  logs  vary  widely  according  to  local 
conditions.  Logging  is  usually  carried  on  in  the  winter. 
During  the  late  fall  and  winter  wood  is  freer  from  sap  and 
if  cut  in  that  condition  decays  less  rapidly  than  if  cut  in  the 
spring  or  early  summer  when  the  sap  is  more  plentiful. 
At  the  saw  mill  the  logs  are  sawed  into  commercial  sizes  of 
lumber,  either  by  rotary  saws  or  by  band  saws.  Poles, 
posts,  and  most  railway  ties  are  usually  hewed  to  shape 
rather  than  sawed. 

Seasoning  of  Timber. — The  high  moisture  content  of 
green  timber  is  reduced  by  exposure  to  atmospheric  air,  or 
by  heating  in  kilns.  The  former  process  is  called  seasoning, 


168 


MATERIALS  OF  ENGINEERING 


TIMBER 


169 


and  reduces  the  moisture  content  from  30  or  35  per  cent, 
to  12  or  15  per  cent.  During  the  period  of  seasoning, 
timber  should  be  piled  so  that  air  has  free  access  to  each 
stick.  The  time  required  for  proper  seasoning  varies 
greatly  for  different  kinds  of  wood,  and  for  different  sized 
pieces,  but  is  never  less  than  several  weeks.  Hard  wood  is 
usually  seasoned  for  several  months  before  being  kiln-dried. 
Kiln-drying  of  timber  is  carried  on  at  a  temperature  of  158° 
to  180°F.  Kiln-drying  reduces  the  moisture  content  of 
timber  to  about  3.5  per  cent.  If  the  timber  is  allowed  to 
get  too  hot,  chemical  changes  take  place  in  the  wood  struc- 
ture which  reduce  the  strength  and  the  resilience  of  the 
wood. 

Shrinkage   of  Timber   during  Seasoning. — During  the 
seasoning  of  wood,  shrinkage  takes  place,  and  the  circum- 


a.  Cracks.  6.  Warping. 

FIG.  67. — Defects  in  wood  caused  by  too  rapid  seasoning  or  kiln-drying. 

ferential  shrinkage  of  a  stick  is  relatively  greater  than  the 
radial  shrinkage.  This  is  due  partly  to  the  resistance  to 
radial  shrinkage  offered  by  the  rays  of  the  wood,  and  partly 
to  the  fact  that  in  a  radial  direction  the  rings  of  summer 
wood  are  resisted  in  their  tendency  to  shrink  by  the  less 
shrinkable  spring  wood,  while  circumferentially  the  sum- 
mer wood  shrinks  along  the  rings.  The  effect  of  this 
unequal  shrinkage  is  to  set  up  internal  stresses  which  some- 
times cause  cracks  or  checks  in  timber  as  illustrated  in 
Fig.  67a. 

In  sawing  boards  or  beams  from  a  log  of  wood  if  the  log 
is  sawed  into  parallel  strips,  as  shown  in  Fig.  68a,  the  boards 


170 


MATERIALS  OF  ENGINEERING 


which  are  cut  squarely  across  the  annual  rings  are  spoken 
of  as  "  quarter-sawed, "  while  the  boards  from  the  edges 
are  spoken  of  as  " slabs."  The  width  of  a  slab  is  nearly 
tangential  to  the  annual  rings,  and  under  the  unequal 
shrinkage  due  to  drying  the  slab  is  warped  much  more  than 
the  quarter-sawed  board.  This  effect  is  shown  in  Fig.  676. 
The  method  of  sawing  shown  in  Fig.  68a  is  'the  common 
method  for  sawing  the  ordinary  grades  of  lumber.  Fig. 
686  and  68c  show  methods  of  cutting  boards  from  a  log  so 
that  slabs  will  be  eliminated  and  at  the  same  time  waste 
minimized. 


a  b  c 

FIG.  G8. — Methods  of  cutting  boards  from  logs. 

Classification  of  Lumber. — The  term  lumber  includes 
all  material  sawn  from  logs,  and  used  for  structural  or 
other  commercial  purposes.  The  large  sizes  of  structural 
members,  beams,  joints,  struts,  etc.,  are  called  timbers. 
Lumber  sawed  on  all  four  sides  is  known  as  "resawed" 
lumber,  and  if  this  resawed  lumber  is  planed  it  is  known  as 
"dressed"  lumber.  The  actual  dimensions  Qf  commercial 
lumber  are  somewhat  less  than  the  nominal  dimensions. 
Lumber  sawed  to  size  is  not  regarded  as  being  " short" 
in  dimension  unless  an  actual  dimension  is  y±  inch  or 
more  under  the  nominal  dimension.  For  dressed  lumber 
an  allowance  of  Y±  inch  for  each  dressed  face  is  made;  a 
stick  nominally  12  in.  by  12  in.,  dressed  on  the  four  sides 
would  actually  measure  about  11^  m-  by  11^2  in. 

The  standard  lengths  for  commercial  lumber  are  multi- 
ples of  two  feet,  from  10  to  24  feet.  The  standard  widths 
for  lumber  are  multiple  of  one  inch. 


TIMBER  171 

Flooring  includes  pieces  1,  1J£,  and  IK  inches  thick, 
by  3  to  6  inches  wide,  excluding  1%  by  6. 

Boards  include  lumber  less  than  1J^  inches  thick  and 
more  than  6  inches  wide. 

The  term  plank  applies  to  any  piece  of  lumber  from  1J^ 
to  (but  not  including)  6  inches  thick  by  6  inches  or  over  in 
width. 

Scantling  includes  all  sizes  over  1J^  and  under  6  inches 
in  thickness  and  from  3  to  6  inches  in  width. 

Dimension  limber  includes  all  sizes  6  inches  or  over  in 
width  by  6  inches  or  over  in  thickness. 

Uses  of  Timber. — At  the  present  time  there  are  used 
annually  in  the  United  States  about  52  billion  board  feet 
of  lumber.1  About  28  per  cent,  of  this  is  used  for  general 
construction  purposes,  26  per  cent,  for  planing  mill  prod- 
ucts,— sash,  doors,  and  general  mill  work, — 9  per  cent,  for 
boxes  and  crates,  9  per  cent,  for  railroad  ties,  5 percent,  for 
mine  timbers,  4  per  cent,  for  pulp  for  paper  manufacture, 
2.5  per  cent,  for  car  manufacture,  2.5  per  cent,  for  shingles, 
and  the  remaining  14  per  cent  for  a  large  variety  of  prod- 
ucts including  furniture,  vehicles,  lath,  veneers,  agricultural 
implements,  chairs,  poles,  ship  building,  etc.  In  addition 
to  the  above  uses  of  lumber  there  are  used  annually  about 
43  billion  board  feet  of  firewood  and  6  billion  board  feet 
of  wood  for  fence  posts  and  rails. 

Structure  of  Wood. — A  tree  grows  by  the  annual  addi- 
tion of  consecutive  rings  of  wood  fiber.  The  growth  takes 
place  on  the  outside  rings.  A  cross-section  of  a  tree  shows 
a  central  core  or  "pith"  of  small  diameter  which  assists 
the  tree  growth  by  storing  up  plant  food  during  the  first 
year  or  two.  Outside  this  pith  are  concentric  rings,  usu- 
ally well  marked,  which  show  the  growth  of  the  tree  from 
year  to  year.  The  width  of  these  rings  varies  widely  in 
different  species  of  trees,  and  in  different  trees  of  the  same 
species  grown  under  different  conditions.  The  width  of 
rings  is  a  function  of  the  rate  of  growth  of  the  tree.  The 

1  The  commercial  measure  of  quantity  of  lumber  is  the  board  foot,  one 
square  foot  of  wood,  one  inch  thick. 


172 


MATERIALS  OF  ENGINEERING 


outer  rings  of  a  mature  tree  serve  as  ducts  for  the  passage 
of  sap,  which  furnishes  the  plant  food  necessary  for  the 
growth  of  the  tree,  and  the  wood  of  the  outer  rings  is  called 
sap  wood  (Fig.  69).  The  inner  rings  have  ceased  to  carry 
sap,  and  the  wood  from  the  inner  rings  is  called  heart  wood. 
In  general,  unless  decay  has  set  in,  the  heart  wood  is  strong- 
er than  the  sap  wood. 

In  the  temperate  zone  the  rate  of  growth  of  a  tree  varies 
greatly  for  different  seasons  of  the  year.  The  growth  is 
most  rapid  in  spring,  less  rapid  in  summer  and  early  fall, 


Wood 


Wood 


•  'Summer  Wood  (Dark) 
FIG.  69.  —  Cross-section  of  tree  showing  annual  rings. 

and  practically  zero  for  late  fall  and  winter.  The  l  i  spring 
wood77  is  usually  lighter  colored  than  the  "summer  wood'7 
and  the  annual  rings  of  a  tree  usually  appear  distinct  on 
account  of  the  juxtaposition  of  the  dark-colored  summer 
wood  and  the  light-colored  spring  wood  of  the  next  year's 
growth. 

The  soft  woods  are  made  up  for  the  most  part  of  an  aggre- 
gation of  elongated  tubular  cells  (tracheids)  which  extend 
in  a  direction  parallel  to  the  axis  of  the  tree  trunk.  These 
cells  are  closed  at  the  ends  and  absorb  water  through  their 
porous  walls.  At  right  angles  to  these  tubular  cells  are 
numerous  groups  of  cells  extending  in  a  radial,  direction 
and  called  rays.  Through  these  rays  plant  food  is  distri- 


TIMBER 


173 


buted  across  the  tree  section.  In  some  soft  woods  there  are 
longitudinal  tubes  called  resin  ducts,  in  which  res-in  is 
formed. 

The  hard  woods  have  a  much  more  complex  structure 
than  the  soft  woods.  In  hard  woods  the  rays  are  much 
larger  and  more  numerous  than  in  soft  woods.  The  elon- 
gated tubular  cells  (tracheids)  are  of  minor  importance,  and 
the  principal  structural  elements  of  the  hard  woods  are 
longitudinal  wood  fibers,  made  up  of  elongated,  sharp- 
pointed  cells  with  very  thick  walls. 


FIG.  70. — The  "grain"  of  wood.     Upper  stick  is  straight-grained,  lower  two 
sticks  show  cross-grained  or  twisted-grained  wood. 

The  general  longitudinal  direction  of  the  tubular  cells 
which  make  up  the  greater  part  of  wood  give  a  distinctive 
" grain"  to  its  structure.  If  these  cells  extend  parallel  to 
the  axis  of  tree  trunk,  the  timber  from  the  tree  is  straight- 
grained,  if  the  fibers  take  a  spiral  course  the  grain  is  twisted, 
and  if  the  " sense"  of  the  spiral  (right-handedness  or  left- 
handedness)  is  reversed  as  the  tree  grows,  the  timber  is 
cross-grained.  Fig.  70  shows  examples  of  straight-grained, 
twisted-grained,  and  cross-grained  timber. 

When,  during  the  growth  of  a  tree,  the  trunk  end  of  a 
branch  becomes  enclosed  by  successive  annual  rings,  there 
is  formed  a  knot  in  the  wood.  Knots  sometimes  con- 
stitute serious  defects  in  timber.  This  will  be  discussed 
later. 


174  MATERIALS  OF  ENGINEERING 

Strength  of  Wood. — The  strength  of  timber  is  affected 
by  a  great  many  factors,  and  for  timber  structures,  the  safe 
working  stresses  are  very  much  lower  than  the  ultimate  as 
shown  by  laboratory  tests  of  specimens;  in  other  words,  it 
is  necessary  to  use  a  high  " factor  of  safety"  for  timber. 
One  of  the  factors  which  influence  the  strength  is  the 
"grain"  of  wood.  The  tensile  strength  and  the  compress- 
ive  strength  in  a  direction  parallel  to  the  grain  (along  the 
grain)  are  much  higher  than  in  a  direction  perpendicular 
to  the  grain  (across  the  grain).  The  shearing  strength 
along  the  grain  is  much  lower  than  the  shearing  strength 
across  the  grain.  The  shearing  strength  of  timber  along 


LOAD 


FIG.  71. — Failure  of  wood  beam  by  horizontal  shear.     Shear  crack  can  be  seen 
extending  along  the  beam  at  about  mid-depth. 

the  grain  is  very  much  lower  than  the  tensile  or  compress- 
ive  strength  along  the  grain,  and  this  fact  must  be  kept  in 
mind  when  the  strength  of  timber  beams  is  being  con- 
sidered. For  this  reason  the  horizontal  shear  in  timber 
beams  is  a  much  more  important  factor  than  it  is  in  steel 
beams,  and  it  must  always  be  taken  into  account  in  design- 
ing such  a  beam.  Fig.  71  shows  a  characteristic  failure  of 
a  timber  beam  by  horizontal  shear.  The  low  shearing 
strength  along  the  grain  makes  it  difficult  to  use  wood  for 
tension  members  of  structures  on  account  of  the  shearing 
stresses  accompanying  tension  stresses.  Fig.  72a  shows  a 
stick  of  timber  in  tension,  held  at  the  ends  by  bolts  through 
the  stick.  For  this  stick  there  is  danger  of  failure  by  shear 
along  the  line  ac  and  bd  rather  than  by  tension  on  a  cross- 


TIMBER 


175 


section  of  the  stick.  To  develop  the  tensile  strength  of  the 
cross-section  it  would  be  necessary  to  have  very  long  ends. 
Fig.  726  shows  a  joint  between  two  timbers.  The  danger 
of  failure  by  shear  along  mn  must  be  considered  as  well  as 
the  compressive  stress  on  nq.  The  tensile  strength  of 
timber  is  of  importance  chiefly  in  the  consideration  of  the 
tensile  stresses  in  timber  beams. 

Timber  makes  good  material  for  compression  members, 
and  it  is  frequently  used  for  such  members.  In  trusses  for 
roofs  and  bridges  it  is  not  uncommon  to  make  the  tension 
members  of  steel  and  the  compression  members  of  timber. 
In  this  connection,  however,  it  must  be  borne  infmind  that 


FIG.  72. — Wooden  structural  parts  under  stress. 

timber  has  a  low  modulus  of  elasticity,  that  is,  its  stiffness 
is  low,  and  its  texture  is  non-uniform,  and  these  facts  make 
long  timber  columns  or  beams  very  sensitive  to  influences 
tending  to  produce  sidewise  buckling.  For  wooden  col- 
umns there  should  be  a  large  reduction  in  the  average 
compressive  unit  stress  to  allow  for  the  effect  of  length  of 
column  upon  allowable  compressive  stress.  Timber  is  very 
rarely  used  for  shear  members  or  for  torsion  members. 
Table  9  gives  average  values  for  the  strength  of  various 
kinds  of  wood,  as  determined  by  laboratory  tests. 

Elastic  Properties  of  Wood. — The  elastic  limit  and  the 
proportional  limit  of  wood  are  less  well  defined  than  the 
corresponding  limits  for  steel,  and  wood  has  no  yield  point. 


176  MATERIALS  OF  ENGINEERING 

The  proportional  limit  furnishes  the  best  criterion  of 
static  strength,  and  values  of  the  proportional  limit  for 
varous  kinds  of  timber  are  given  in  Table  9,  which  also 
gives  values  of  the  modulus  of  elasticity  for  timber.  The 
ratio  of  strength  to  stiffness  is  much  higher  for  timber  than 
for  iron  or  steel,  and  very  much  higher  than  for  cast  iron  or 
concrete. 

Wood  is  a  good  material  for  structural  members  and 
machine  parts  which  must  withstand  shock,  such  as  railway 
ties,  fence  posts,  spokes  and  rims  of  wheels,  hammer  handles, 
etc.  The  reason  for  this  is  the  fact  that  resistance  to  shock 
depends  on  two  factors :  (1)  the  stress  which  can  be  carried, 
and  (2)  the  deformation  (stretch,  compression,  or  deflec- 
tion) which  can  be  withstood.  Wood  can  not  withstand  a 
high  unit  stress  without  injury,  but  can  withstand  a  very 
large  deformation  without  injury.  The  elastic  resistance 
to  impact  (resilience)  for  any  material  is  measured  by  the 
area  under  the  stress-strain  diagram  up  to  the  proportional 
limit,  and  an  examination  of  stress-strain  diagrams  for 
wood,  cast  iron,  and  steel  shows  that  for  elastic  resistance 
to  shock,  wood  has  about  half  the  capacity  per  cubic  inch 
of  steel,  and  a  much  higher  capacity  per  cubic  inch  for  elastic 
resistance  to  shock  than  does  cast  iron  (see  Fig.  15,  page  31 
•for  typical  stress-strain  diagrams  for  steel,  cast  iron  and 
wood).  Wooden  parts  for  structures  and  machines  have, 
in  general,  ten  or  twelve  times  the  volume  of  steel  or  cast- 
iron  parts,  in  order  to  resist  the  stresses  set  up,  and  as 
shock-resisting  capacity  of  a  member  is  proportional  to 
the  product  of  its  volume  and  the  shock-resisting  capacity 
per  cubic  inch  of  the  material,  wooden  structural  members 
have  a  higher  capacity  for  elastic  resistance  to  shock  than 
do  steel  members,  and  a  very  much  higher  capacity  than 
do  cast-iron  members. 

In  resistance  to  complete  rupture  under  shock  wood  ranks 
higher  than  does  cast  iron,  but  lower  than  structural  steel. 
Resistance  per  cubic  inch  of  material  to  complete  rupture 
under  shock  is  measured  by  the  area  under  the  entire  stress- 
strain  diagram.  An  examination  of  Fig.  14,  page  30,  gives 


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178  MATERIALS  OF  ENGINEERING 

some  idea  of  the  relative  capacities  of  wood,  steel  and  cast 
iron  for  resistance  to  complete  rupture  under  shock. 

Strength  of  Large  Pieces  of  Timber. — In  using  the  values 
of  ultimate  strength  given  in  Table  9  as  the  basis  from 
which  to  determine  allowable  working  stresses  for  full- 
sized  wooden  structural  members  the  effect  of  size  of  mem- 
ber must  be  considered.  If  the  structural  member  is  small, 
e.g.,  a  rod  for  the  frame  of  an  aeroplane,  and  a  selected 
piece  of  clear,  straight-grained  timber  can  be  used,  the 
values  for  ultimate  strength  given  in  Table  9  for  small, 
clear  test  specimens  could  be  developed.  For  a  large  struc- 
tural member  of  the  same  kind  of  wood,  e.g.,  for  a  bridge 
stringer,  the  probability  of  knots,  cross-grained  wood,  and 
other  defects  would  be  high,  and  the  ultimate  fiber  stress 
would  be  that  given  for  full  sized  structural  timbers.  Tests 
of  large  timber  beams  and  columns  always  give  lower  re- 
sults for  ultimate  stress  than  do  tests  of  small  selected 
specimens  of  the  same  kind  of  wood.  Table  4  gives  work- 
ing stresses  for  various  kinds  of  timber,  which  have  come 
to  be  considered  safe  by  engineers. 

Effect  of  Moisture  on  the -Strength  of  Timber. —  Struc- 
tural timber  as  ordinarily  placed  on  the  market  contains 
about  12  per  cent,  of  water.  If  the  timber  has  been  kiln- 
dried,  the  moisture  content  is  reduced  to  about  3.5  per 
cent.  Green  timber  may  contain  as  high  a  moisture  con- 
tent as  35  per  cent.  Up  to  a  moisture  content  of  about 
25  per  cent,  the  fibers  of 'the  wood  absorb  water  and  are 
softened  and  weakened  by  it,  especially  in  compression. 
Above  this  saturated  condition,  which  is  called  the  "  fiber 
saturation  point "  water  no  longer  affects  the  strength  and 
stiffness  of  wood.  The  general  effect  of  moisture  content 
on  the  strength  and  the  stiffness  of  soft  wood  is  shown  by 
Fig.  73,  the  basis  of  which  is  found  in  the  results  of  tests 
by  the  U.  S.  Forest  Service,  made  principally  by  Mr.  H.  D. 
Tiemann.  As  an  illustration  of  the  use  of  Fig.  73  consider 
the  flexural  strength  of  wood  which,  when  in  a  green  or 
saturated  condition  has  a  moisture  content  of  25  per  cent. 
If  such  wood  is  air-dried  until  its  moisture  content  is  re- 


TIMBER 


179 


duced  to  12  per  cent,  it  is  then  l%5  or  48  per  cent,  satu- 
rated, and  its  flexural  strength  will  be  increased  to  about 
6?1j9  or  154  per  cent,  of  its  strength  green.  If  it  is  kiln- 
dried  until  its  moisture  content  is  reduced  to  3  per  cent,  it 
will  be  %5  or  12  per  cent,  saturated  and  its  flexural  strength 
will  be  inci  eased  to  about  9%9  or  231  per  cent,  of  its 
strength  green. 


Based  on  Test  Data  in  Bulletins 
of  U  5  Forest  Service 


0 


10 


20       30       40        50        60       70       80 
Water,  Per  Cent,  of  fiber  Saturation 

FIG.  73.— Effect  of  moisture  on  strength  and  stiffness  of  soft  wood.  For 
soft  wood  fiber  saturation  occurs  at  a  moisture  content  averaging  about  25  per 
cent. 

Time  Element  in  the  Strength  of  Timber. — Under  long- 
continued  static  loads  timber  will  fail  with  fiber  stresses 
much  lower  than  the  ultimate  stresses  found  by  laboratory 
tests,  in  which  the  loading  extends  over  a  few  minutes. 
This  time  effect  extends  over  several  months,  if  not  over 
longer  periods  of  time,  for  large  pieces  of  timber.  Under 
long-time  loading,  test  pieces  of  timber  have  been  broken 
under  stresses  a  little  greater  than  50  per  cent,  of  the  util- 


180  MATERIALS  OF  ENGINEERING 

mate  strength  as  given  by  short-time  tests.     This  very  pro- 
nounced time  effect  is  one  of  the  reasons  why  working 
stresses  for  timber  are  so  low  as  compared  with  the  util- 
mate  strength  as  determined  by  tests  in  a  testing  machine. 
Relation  of  Strength  and  Shrinkage  of  Timber  to  Den- 
sity.— An  examination  of  a  large  number  of  test  results, 
made  by  Newlin  and  Wilson  of  the  U.  S.  Forest  service 
have    established    fairly   definite    relations    between    the 
strength  and  the  shrinkage  properties  of  wood  and  its  spe- 
cific gravity.     In  their  investigation  they  determined  the 
specific  gravity  as  the  ratio  of  the  weight  of  a  specimen  of 
wood  oven-dry  to  the  weight  of  a  volume  of  water  equal  to 
the  volume  of  the  specimen  at  the  time  of  testing.     Calling 
this  ratio  G  the  following  equations  give  the  shrinkage  in 
per  cent  from  green  condition  to  oven-dry  condition. 
Shrinkage  in  volume,  per  cent  =  28  G 
Shrinkage  in  radial  dimensions,  per  cent  =  .9.5  G 
Shrinkage  in  tangential  dimensions,  per  cent  =  17.0  G 
Table  10  gives  the  relations  of  strength  values  established 
by  the  study  of  test  data. 

Common  Defects  in  Timber. — Some  of  the  common  de- 
fects found  in  commercial  timber  are:  cross-grain,  knots, 
splits  and  cracks  (called  " shakes")?  bark  or  raggedness  of 
wood  at  the  edges  of  boards  (called  "wane"),  and  pitch 
pockets.  The  general  requirements  for  yellow  pine  bridge 
and  trestle  timbers  as  given  by  the  1918  " Standards" 
of  the  American  Society  for  Testing  Materials  furnish  a 
good  example  of  reasonable  requirements  for  structural 
timber.  These  general  requirements  read,  in  part : 

"Except  as  noted  all  timber  shall  be  cut  from  sound  trees  and  sawed 
standard  size;  close-grained  and  solid;  free  from  defects  such  as  injurious 
ring  shakes  and  crooked  grain;  unsound  knots;  knots  in  groups;  decay; 
large  pitch  pockets,  or  other  defects  that  will  materially  impair  its 
strength." 

An  explanation  of  some  of  the  terms  used  in  the  foregoing 
paragraph  may  be  of  service.  Ring  shakes  are  cracks  be- 
tween the  annual  rings  of  the  wood.  " Unsound"  knots 
include  loose  knots  and  rotten  knots  (knots  softer  than  the 


TIMBER  181 

TABLE  10. — RELATION  OF  STRENGTH  OF  WOOD  TO  ITS  SPECIFIC  GRAVITY 

Based  on  the  investigation  of  Newlin  and  Wilson  of  the  U.  S.  Forest 
Service,  see  Bulletin  676,  U.  S.  Dept.  of  Agriculture. 

G  is  the  specific  gravity  of  the  wood,  based  on  the  weight  oven-dry  and 
the  volume  at  the  time  of  test  and  in  the  condition  when  tested. 


Strength 

property 

Green  wood 

Airy-dry  wood 

Static  flexure: 

Unit  stress  at  proportional  limit,  Ib.  per 

sq.  in 

Modulus  of  rupture,  Ib.  per  sq.  in.1 

Modulus  of  elasticity,  Ib.  per  sq.  in 

Impact  flexure  with  50-lb.  hammer  unit*2 

stress  at: 
Proportional  limit,  Ib.  per  sq.  in 


1 0,300v/(?5 
18,500-^ 
2,500,000  G 


23,500v/G5 


Modulus  of  elasticity,  Ib.  per  sq.  in 3,000,000  G 


Compression  parallel  to  grain : 

Proportional  limit,  Ib.  per  sq.  in 

Unit  stress  at  ultimate,  Ib.  per  sq.  in. 

Modulus  of  elasticity,  Ib.  per  sq.  in. .  . 
Compression  perpendicular  to  grain: 

Proportional  limit,  Ib.  per  sq.  in 


Shear  along  grain: 

Unit  stress  at  ultimate,  Ib.  per  sq.  in. . 


6,800  \/G5 
6,900  G 
2,860,000  G 

2,900-v/G^ 


26,200>/G& 
3,000,000  G 


35,000-^ 
3,550,000  G 


12,000  G 
3,500,000  G 

5,200-^(79 


1  See  p.  19  for  definition  of  modulus  of  rupture. 

2  See  p.  281  for  description  of  impact  test. 

surrounding  wood).  Pitch  pockets  are  openings  between 
the  fibers  of  the  wood,  extending  along  the  grain,  and  con- 
taining pitch  or  bark.  A  pitch  pocket  more  than  %  in. 
wide  or  3  in.  long  is  considered  a  large  pitch  pocket. 

The  Grading  of  Lumber. — Lumber  is  graded  according 
to  the  number,  size,  and  location  of  the  visible  defects  it 
contains.  For  some  purposes  clear  lumber,  that  is  lumber 
without  visible  defects,  is  necessary,  while  for  other  purposes 
lumber  with  slight  defects  will  serve.  For  some  purposes, 
such  as  flooring,  lumber  is  graded  by  its  best  side,  while  for 
other  purposes,  such  as  door  stock,  the  lumber  must  be  free 
from  defects  on  both  sides. 

Rules  for  grading  lumber  are   generally  prepared  by 


182  MATERIALS  OF  ENGINEERING 

saw  mills  associations,  or  lumber  manufacturers'  associa- 
tions covering  a  certain  territory,  an  issuing  rules  for  grad- 
ing certain  species  of  lumber. 

Veneer,  Plywood. — Owing  to  the  increasing  cost  of  hard 
wood  there  has  been  developed  the  use  of  boards  of  soft 
wood  with  very  thin  sheets  of  hard  wood,  glued  on.  These 
thin  sheets  are  known  as  veneer,  and  have  been  very 
commonly  used  to  give  a  hard  wood  surface  to  cheap 
lumber. 

Quite  recently  the  idea  of  built-up  lumber  has  been  ex- 
tended to  the  development  of  plywood, — wood  built  up 
of  this  layers  with  the  grain  of  successive  layers  at  right 
angles,  and  all  layers  glued  together  with  waterproof  glue. 

Plywood  thus  made  has  two  advantages  over  ordinary 
1  umber :  ( 1 )  the  tendency  of  any  layer  to  shrink  transversely 
to  the  grain  under  change  of  moisture  content  is  resisted  on 
account  of  the  direction  of  the  grain  of  the  adjacent  layers, 
and  (2)  the  net  shrinkage  of  the  built-up  lumber  is  only 
slightly  greater  than  the  shrinkage  of  ordinary  lumber 
parallel  to  the  grain. 

The  tensile  strength  of  wood  across  the  grain  is  so  small 
as  to  be  negligible.  The  arrangement  of  the  layers  of 
plywood  gives  tensile  strength  in  all  directions,  the  tensile 
strength  of  plywood  being  about  half  that  of  ordinary 
lumber  along  the  grain.  The  arrangement  of  the  layers  of 
plywood  also  diminishes  the  tendency  characteristic  of 
ordinary  lumber  to  shear  along  the  grain.  Plywood  is  a 
material  of  nearly  uniform  strength  in  different  directions 
having  a  strength  about  equal  to  the  average  of  the  strength 
of  ordinary  lumber  with  the  grain  and  the  strength  across 
the  grain.  As  plywood  is  built  up  of  thin  layers  it  becomes 
feasible  to  use  the  highest  grade  of  wood  in  its  manufacture. 

Plywood  is  always  built  up  of  an  odd  number  of  piles, 
so  that  the  shrinkage  stresses  will  be  symmetrical  about  the 
middle  layer,  and  there  will  be  no  tendency  for  them  to 
warp  the  wood.  Plywood  may  be  built  up  of  a  few  layers 
each  conparatively  thick,  or  of  a  larger  number  of  thinner 
layers.  In  general  the  plywood  built  up  of  the  large  num- 


TIMBER 


183 


ber  of  layers  is  moren  early  homogeneous  iri  its  elastic 
action,  but  is  more  expensive  to  make. 

Bass  wood,  redwood,  poplar,  maple,  birch,  red  gum  are 
domestic  woods  which  can  be  cut  into  the  thin  veneers 
necessary  in  making  plywood,  and  from  which  satisfactory 
plywood  can  be  made.  Basswood,  redwood,  and  poplar 
veneers  are  not  used  for  face  veneers  in  making  plywood. 
Table  11  gives  values  for  the  tensile  strength  of  three-ply 
plywood  made  of  various  kinds  of  wood. 

When  a  structure  can  be  so  designed  that  timber  can  be 
used  as  the  material  without  danger  of  failure  by  longi- 
tudinal shear  or  by  cross-grain  tension,  no  other  material 
will  in  general,  give  so  light  a  structure.  The  use  of  ply- 
wood extends  the  possibilities  of  using  the  lightness  of 
timber  construction,  by  lessening  the  danger  of  splitting 
or  shearing  along  the  grain. 

Plywood  is  already  of  importance  in  the  manufacture  of 
motor  cars,  street  and  railway  cars,  airplanes  and  boats,— 
all  structures  in  which  it  is  important  to  have  minimum 
weight  consistent  with  safety. 

It  is  evident  that  the  strength  of  plywood  will  be  de- 
pendent on  the  strength  of  the  glue  joint  between  layers 
of  veneer. 

TABLE  11.— TENSILE  STRENGTH  OF  PLYWOOD 

Based  on  tests  at  the  U.  S.  Forest  Products  Laboratory,  Madison,  Wis. 


Species 

Weight  of 
plywood, 
kiln-dry, 
Ib.  per  cu.  ft. 

Moisture 
at  test 
per  cent. 

Ultimate  tensile 
strength  of  three-ply 
plywood,  parallel  to 
grain  of  faces,2 
Ib.  per  sq.  in. 

Ultimate  tensile 
strength  of  single-ply 
veneer,  parallel  to 
grain, 
Ib.  per  sq.  in. 

Basswood  

28 

9.2 

6,900 

10,300 

Yellow  Birch.  . 

45 

85 

13,200 

19,800 

Maple,  soft  .  .  . 

38 

8.9 

8,200 

12,300 

hard  .  . 

46 

8.0 

10,200 

15,300 

Yellow  Poplar. 

34 

3.4 

7,400 

11,100 

Red  gum 

36 

8.7 

7,800 

11  800 

Redwood  

28 

9.7 

4,800 

7,200 

1  About  8  per  cent,  moisture. 

2  Strength  values  given  are  for  three-ply  wood  with  plies  of  same  thick- 
ness and  same  species:  values  for  plywood  of  different  make-up  would 
not  be  the  same  as  those  given  in  the  table. 


184  MATERIALS  OF  ENGINEERING 

Decay  of  Wood. — The  cells  of  wood  with  the  water 
found  in  them  furnish  food  for  a  variety  of  destructive 
bacteria  and  fungi.  These  bacteria  and  fungi  feed  on  the 
moist  wood  fiber  and  cause  rotting  of  the  wood.  Lack  of 
moisture  diminishes  the  food  value  of  the  wood,  and  hence 
seasoning,  which  removes  moisture,  diminishes  the  rapidity 
of  decay  of  wood  and  lengthens  its  life.  Well-seasoned 
timber  which  is  not  exposed  to  moisture  retains  its  strength 
for  many  years.  The  usefulness  of  the  seasoning  process 
in  preserving  the  life  of  timber  structures  which  are  exposed 
to  moisture  is  limited  by  the  fact  that  in  such  structures 
the  wood  soon  reabsorbs  moisture,  and  again  furnishes 
abundant  food  for  the  destructive  bacteria  and  fungi.  In 
this  connection  it  should  be  noted  that  timber  kept  con- 
tinuously under  water  does  not  decay,  though  in  sea  water 
it  may  be  attacked  by  marine  boring  animals,  such  as  the 
teredo. 

Preservatives  for  Timber. — The  decay  of  timber  exposed 
to  moisture  can  be  very  greatly  retarded  if  the  fibers  are 
impregnated  with  some  substance  which  is  poisonous  to  the 
decay-causing  bacteria  and  fungi.  An  ideal  preservative 
for  timber  would  be  poisonous  to  decay-causing  organism, 
capable  of  being  injected  into  the  innermost  fibers  of 
the  pieces  of  timber  treated,  would  be  retained  in  the  wood, 
and  would  be  cheap.  The  two  wood  preservatives  in  com- 
mon use  are  creosote  and  zinc  chloride.  Creosote  is  an  oil, 
a  product  of  coal-tar  distillation.  It  is  poisonous  to 
wood-destroying  bacteria,  it  is  not  soluble  in  water,  and 
hence  will  not  be  dissolved  out  of  timber  of  the  action  of 
rains  or  of  flowing  streams.  It  can  be  forced  into  the  inner 
fibers  of  soft  woods  and  of  some  hard  woods.  Zinc  cholride 
is  violently  poisonous  to  timber-destroying  bacteria,  can  be 
readily  forced  into  the  inner  fibers  of  wood,  and  is  cheaper 
than  creosote.  It  is,  however,  soluble  in  water,  and  is 
gradually  dissolved  out  of  timber  which  is  exposed  to 
water. 

Preservative  Processes  for  Timber. — The  simplest  proc- 
ess of  treating  timber  to  preserve  it  against  decay  consists 


TIMBER  185 

in  simply  soaking  the  pieces  to  be  treated  in  an  open  tank 
of  hot  creosote  or  other  preservative.  This  process  does 
not  impregnate  the  timber  very  thoroughly,  and  it  is  waste- 
ful of  preservative.  This  "open-tank"  process  is  used 
only  for  treating  small  lots  of  timber  where  the  apparatus 
for  more  thorough  treatment  is  not  available. 

The  general  method  followed  in  the  commercial  treatment 
of  timber  either  with  zinc  chloride,  or,  as  is  more  common, 
with  creosote  involves  the  following  steps;  (1)  seasoning 
the  timber;  (2)  steaming  the  timber  in  a  large  cylinder  to 
soften  the  wood  fiber;  (3)  the  removal  of  air  and  moisture 
from  the  interior  of  the  cylinder  and  from  the  wood  fibers 
by  means  of  a  vacuum  pump;  (4)  the  connection  of  the 
interior  of  the  cylinder  with  a  tank  of  preservative  (creosote 
or  zinc  chloride),  the  preservative  rushes  into  the  partial 
vacuum  formed  in  the  cylinder  and  penetrates  some  dis- 
tance into  the  wood  structure;  (5)  the  application  of 
pressure  to  the  cylinder  forcing  the  preservative  into  the  in- 
nermost fibers  of  the  timber;  and  (6)  the  removal  of  the 
pressure,  after  which  the  excess  of  preservative  is  allowed 
to  drip  off  the  timber  and  run  into  a  tank. 

Uses  of  Treated  Timber. — The  treatment  of  railway 
ties  to  preserve  them  against  decay  is  the  most  widely 
developed  application  of  the  timber  treating  process.  Usu- 
ally the  tie-treating  process  is  carried  on  in  a  large  plant, 
in  which  several  cylinders  about  150  ft.  long  by  10  ft.  in 
diameter  receive  a  number  of  small  cars  loaded  with  ties 
to  be  treated. 

Timber  for  piles,  especially  for  piles  which  are  to  stand 
in  salt  water,  is  also  frequently  treated  with  preservative. 
Poles  for  carrying  electric  wires,  and  fence  posts  are  some- 
times treated  by  the  open-tank  process  over  the  ends 
which  are  to  be  placed  in  the  ground. 

The  life  of  a  soft  wood  railway  tie  untreated  with  pre- 
servatives varies  from  5  to  8  years,  creosoted  it  will  re- 
sist decay  for  10  to  14  years.  If  railway  ties  are  to  be  used 
in  a  very  dry  location,  zinc  chloride  will  be  almost  as 
effective  as  creosote  in  lengthening  the  life  of  the  tie. 


186  MATERIALS  OF  ENGINEERING 

Strength  of  Treated  Timber. — The  effect  of  the  preserva- 
tive processes  on  the  strength  of  timber  has  been  the  subject 
of  much  discussion,  and  several  series  of  tests  on  the  rela- 
tive strength  of  treated  timber  and  untreated  timber  have 
been  made.  Injury  may  be  done  to  the  timber  if  the 
preservative  process  is  carelessly  carried  out,  especially 
is  there  danger  of  injuring  the  timber  by  excessive  pressure 
while  it  is  being  steamed.  If  the  preservative  process  is 
carefully  carried  out,  tests  seem  to  indicate  that  the 
strength  of  the  timber  is  but  little  impaired,  if  at  all. 

Selected  References  for  Further  Study 

SNOW:  "The  Principal  Species  of  Wood,  Their  Characteristic  Properties," 
New  York,  1917. 

RECORD:  "Identification  of  the  Economic  Woods  of  the  United  States," 
New  York,  1912. 

RECORD:  "Mechanical  Properties  of  Wood,"  New  York,  1914. 

ROTH:  Timber,  An  Elementary  Discussion  of  the  Characteristics  and  Prop- 
erties of  Wood,  U.  S.  Department  of  Agriculture,  Forestry  Division, 
Bulletin  10.  Also  reprinted  as  Chap.  XIII  of  "The  Materials  of  Con- 
struction," by  J.  B.  JOHNSON.  Fourth  edition. 

TIEMANN:  The  Strength  of  Wood  as  Influenced  by  Moisture,  U.  S.  Forest 
Service  Bulletin. 

HATT:  Experiments  on  the  Strength  of  Treated  Timber,  U.  S.  Forest  Service 
Bulletin. 

TALBOT:  Tests  of  Timber  Beams,  University  of  Illinois,  Engineering  Experi- 
ment Station  Bulletin  41. 

NEWLIN  AND  WILSON:  "Mechanical  Properties  of  Woods  in  the  United 
States,"  U.  S.  Department  of  Agriculture,  Bulletin  556. 

NEWLIN  AND  WILSON:  "The  Relation  of  the  Shrinkage  and  Strength  Prop- 
erties of  Wood  to  its  Specific  Gravity,"  U.  S.  Department  of  Agricul- 
.  ture,  Bulletin  676. 

BETTS:  "Timber,  Its  Strength,  Seasoning,  and  Grading,"  New  York,  1919. 

MARKWARDT  AND  ELMENDORF:  "Mechanical  Test  Made  on  Plywood." 
Hardwood  Record,  July  10,  1919. 

ELMENDORF:  "Factors  Affecting  Warping  of  Plywood,  "Hardwood  Record, 
July  23,  1919. 

BASQUIN:  "What  is  Haskelite,"  Trade  publication  of  the  Haskelite  Manu- 
facturing Corporation,  Chicago,  111.  pamphlet  describing  a  commercial 
plywood. 


CHAPTER  XVI 
STONE,  BRICK  AND  TERRA-COTTA 

General  Uses  of  Building  Stone. — Since  the  earliest 
known  times  stone  has  been  used  as  a  material  of  construc- 
tion for  walls,  foundations,  and  dams.  Stone  arches  have 
been  in  use  for  many  centuries.  Today  the  use  of  stone 
masonry  for  purely  structural  purposes  is  of  diminishing 
importance  owing  to  the  great  development  of  reinforced 
concrete,  but  building  stone  is  still  used  for  buildings  and 
structures  in  which  appearance  and  permanence  are  large 
factors  in  design.  Of  the  stone  quarried  in  the  United 
States  about  50  per  cent,  is  used  for  structural  purposes, 
the  remainder  being  used  for  crushed  stone  for  roads, 
railroad  ballast,  and  concrete. 

Varieties  of  Building  Stone. — The  common  building 
stones  include  granite,  Imestone  (including  marble), 
sandstone,  and  slate;  there  are  many  varieties  of  each  of 
these  general  classes.  The  granites  are  the  hardest  and 
strongest  building  stones  in  general  use,  the  sandstones 
are  next  in  hardness  and  strength,  and  the  limestones  are, 
in  general,  the  softest  and  weakest  (though  the  stronger 
limestones  are  stronger  than  the  weaker  sandstones). 
The  use  of  slate  is,  in  general,  confined  to  roofing  and  some 
interior  work  in  buildings.  The  harder  and  stronger  a 
building  stone  is,  the  more  difficult  and  expensive  it  is  to 
quarry  blocks  of  that  stone  and  to  dress  them  to  shape. 
In  general,  the  harder  stones  are  the  more  durable.  The 
life  of  American  stone  structures  before  disintegration 
under  weathering  (the  action  of  frost,  chemical  action 
from  gases,  etc.)  is  estimated  to  vary  from  about  12  years 
for  soft  sandstone  to  several  centuries  for  the  harder 
granites. 

187 


188  MATERIALS  OF  ENGINEERING 

Stone  Quarrying  and  Stone  Cutting. — Blocks  of  building 
stone  are  cut  or  blasted  from  the  ledges  of  rock  which  con- 
stitute a  stone  quarry.  These  rough  blocks  are  dressed  to 
shape  in  stone  sheds,  usually  located  near  the  quarries. 
The  squaring  and  final  shaping  of  the  stone  is  done  by  cut- 
ting tools  operated  either  by  hand,  or  more  commonly,  by 
compressed  air.  Power-driven  planers,  lathes,  and  saws 
for  shaping  large  pieces  of  stone  are  used  to  a  considerable 
extent,  especially  for  shaping  marble. 

Masonry  Construction. — In  laying  together  individual 
pieces  of  stone  to  form  masonry,  various  methods  are 
followed.  The  simplest  form  of  masonry  is  riprap, .  which 
consists  of  uncut  stones  piled  together  without  any  adhe- 
sive mortar  between  them.  Riprap  is  used  for  protective 
embankments  for  streams,  and  for  low  stone  walls.  It  has 
very  little  structural  strength,  and  is  not  used  for  struc- 
tures subjected  to  any  great  amount  of  load.  Rubble 
masonry  is  made  up  of  uncut  stones  piled  and  cemented 
together  with  a  matrix  of  mortar.  In  uncoursed  rubble  the 
stones  are  piled  without  any  attempt  at  regularity  of 
arrangement;  in  coursed  rubble  the  stones  are  piled  in 
layers  as  regularly  arranged  as  possible.  Squared  stone 
masonry  is  built  up  of  blocks  of  stone  dresssed  to  regular 
shapes.  Cut  stone  masonry  or  Ashlar1  masonry  is  built 
up  of  blocks  of  stone  squared  and  with  the  faces  dressed  to 
a  fairly  smooth  surface. 

Strength  of  Stone  and  of  Stone  Masonry. — Stone 
masonry  is  used  in  structures  principally  to  lesist  compress- 
ive  stress.  Individual  blocks  of  stone  are  sometimes  used 
to  resist  bending,  such  as  the  lintels  for  windows  and  doors, 
or  as  top  slabs  for  culverts.  The  strength  per  square  inch 
of  specimens  of  stone  is  very  much  greater  than  the  strength 
per  square  inch  of  masonry  built  from  that  kind  of  stone, 
on  account  of  the  presence  in  masonry  of  mortar  joints, 
which  are  weaker  than  the  stone.  Strong  stone,  in  general, 
makes  stronger  masonry  than  does  weak  stone,  and  on  this 

1  By  some  authorities  the  term  Ashlar  is  used  only  for  cut  stone  masonry 
in  which  the  joints  are  not  more  than  }$  in.  thick. 


STONE,  BRICK  AND  TERRA-COTTA 


189 


account,  as  well  as  on  account  of  the  occasional  use  of  indi- 
vidual stone  slabs  in  flexure,  the  strength  of  different  kinds 
of  stone  is  of  importance  to  the  structural  engineer.  The 
actual  strength  of  stone  masonry  depends  largely  on  the 
strength  of  the  mortar  used  and  on  the  closeness  of  fit 
between  adjacent  stones.  Ashlar  masonry  is  about  seven 
times  as  strong  as  uncoursed  rubble  masonary. 

TABLE  12. — VALUES  FOR  STRENGTH  AND  STIFFNESS  OF  AMERICAN  BUILDING 

STONE 

Values  based  mainly  on  test  data  from  the  Watertown  (Mass.)  Arsenal 


Modulus 

Modulus  of  rupture 

Ultimate 

of 

Stone 

Ultimate  in  compres- 
sion, Ib.  pers  q.  in. 

(computed  ultimate  in 
cross-bending), 
Ib.  per  sq.  in. 

in  shear, 
Ib.  per 
sq.  in., 

elasticity 
(flexure), 
Ib.  per 
sq.  in., 

Weight 
(av.), 
Ib.  per 
cu.  ft. 

Range 

Av. 

Range 

Av. 

Av. 

Av. 

Granite  .  .  . 

15,000-26,000 

20,200 

1,200-2,200 

1,600 

2,300 

7,500,000 

165 

Marble  .  .  . 

10,300-16,100 

12,600 

850-2,300 

1,500 

1,300 

8,200,000 

170 

Limestone  . 

3,200-20,000 

9,000 

250-2,700 

1,200 

1,400 

8,400,000 

160 

Sandstone. 

6,700-19,000 

12,500 

500-2,200 

1,500 

1,700 

3,300,000 

135 

Slate  

15,000 

8,500 

14,000,000 

175 

The  compressive  strength,  the  flexural  strength,  and  the 
strength  in  shear  of  common  American  building  stones  are 
given  in  Table  12.  Allowable  loads  on  different  kinds 
of  stone  masonry  are  given  in  Table  4. 

Burnt-clay  Products — Brick,  Terra-cotta  and  Tile.— 
An  important  class  of  materials  of  construction  comprises 
products  of  clay-burning  kilns.  The  burnt-clay  products 
include  building  brick,  paving  brick,  firebrick,  terra-cotta 
blocks  and  tiles,  porcelains,  drain  pipe  and  sewer  pipe. 
Clay  suitable  for  making  common  building  brick  is  very 
common,  and  large  deposits  are  found  in  many  locations. 
Special  grades  of  clay  are  required  for  making  paving  brick, 
for  firebrick,  for  terra-cotta,  and  for  porcelain. 

General  Process  of  Brick -making. — The  clay  is  first 
washed  to  free  if  from  pebbles,  soil,  or  excessive  amounts 
of  sand;  it  is  then  ground  fine  and  mixed  with  water,  after 
which  the  mixture  is  reduced  to  a  plastic  mass  in  a  "pug- 


190  MATERIALS  OF  ENGINEERING 

•mill, "  which  consists  of  a  horizontal  cylinder  in  which  re- 
volving blades  slice  up  the  clay,  mix  it  thoroughly,  and 
finally  force  it  out.  Brick-making  processes  are  classified 
as  stiff-mud,  soft-mud,  or  semi-dry  according  to  the  degree 
of  plasticity  of  the  clay  used.  After  treatment  in  the  pug- 
mill  the  plastic  mass  of  clay  is  molded  into  bricks,  tiles,  pipe 
sections,  or  blocks.  For  tiles,  sewer  pipe,  drain  pipe,  and 
hollow  building  blocks  this  molding  is  usually  done  in 
metal  molds  under  heavy  pressure.  For  bricks  the  molding 
is  sometimes  done  in  molds,  but  the  method  commonly 
used  consists  in  forcing  through  a  die  a  ribbon  of  clay  with 
a  cross-section  of  the  size  of  a  brick.  From  this  ribbon 
bricks  are  cut  off  by  means  of  wires.  The  molded  bricks, 
blocks,  or  tile  are  dried  for  a  period  varying  from  a  few 
•hours  to  several  days,  after  which  they  are  burned  in  kilns 
for  about  a  week.  After  burning,  the  bricks  are  allowed 
to  cool  slowly  in  the  kiln  for  a  period  of  several  days. 

Classification  of  Building  Brick. — The  bricks  from  dif- 
ferent parts  of  a  brick  kiln  vary  markedly  in  quality.  Arch 
brick  or  hard  brick  are  those  which  from  their  position  in 
the  kiln  have  been  overburned;  they  are  apt  to  be  warped 
out  of  shape  by  the  excessive  burning;  red  brick  or  well- 
burned  brick  make  up  the  standard  output  of  a  brick  kiln; 
salmon  brick  or  soft  brick  are  underburned  brick,  which  are 
weak  and  unsuitable  for  use  except  for  masonry  filling. 
Pressed  brick  (repressed  or  face  brick)  are  brick  which  .after 
drying  and  before  burning  are  subjected  to  heavy  pressure 
in  molds.  This  pressure  makes  the  brick  more  nearly 
perfect  in  shape.  It  also  rounds  the  corners.  Pressed 
bricks  are  much  more  costly  than  common  bricks  and  are 
used  mainly  where  appearance  is  of  great  importance. 
The  standard  size  of  building  brick  is  2  in.  by  4  in.  by  8  in. 

Paving  Brick  and  Firebrick. — Paving  bricks  are  usually 
made  from  shale  (clay  hardened  to  rock-like  structure) 
and  in  the  process  of  manufacture  a  higher  temperature  of 
burning  is  used  than  in  making  building  brick.  This 
temperature  for  paving  brick  is  so  high  that  the  burned 
clay  has  a  slightly  vitreous  (glassy)  surface.  The  standard 


STONE,  BRICK  AND  TERRA-COTTA  191 


paving  brick  is  8  to  9  in.  long,  3^  to  3%  in.  wide,  and 
to  4^  in.  thick. 

Firebrick  capable  of  resisting  high  temperatures  may 
be  divided  into  acid,  basic  and  neutral  firebrick.  Acid 
firebricks,  used  for  such  purposes  as  the  lining  of  acid-steel 
furnaces  are  made  from  selected  fireclay  or  from  a  mixture 
of  a  silica-bearing  material  (sand  or  ganister)  and  lime. 
Acid  firebricks  are  burned  at  a  very  high  temperature. 
Basic  firebricks  used  for  the  lining  of  basic-steel  furnaces 
are  made  from  clay  mixed  with  some  substance  containing 
magnesium  or  aluminum  (magnesia  or  bauxite).  Basic 
firebricks  are  also  burned  at  a  very  high  temperature. 
Neutral  firebricks  are  made  of  a  mixture  of  chrome  iron 
and  fireclay. 

Terra  -cotta.  —  Terra-cotta  is  made  in  the  same  general 
way  as  is  brick.  The  raw  material  is  carefully  selected 
clay.  Hard  terra-cotta  cuilding  blocks,  fireproofing  ma- 
terial, and  tile  are  made  by  burning  the  clay  at  such  a 
high  temperature  that  the  resulting  product  has  a  slightly 
vitrified  surface.  Hard  terra-cotta  is  a  strong,  brittle 
material.  Porous  terra-cotta,  sometimes  called  terra- 
cotta lumber,  is  made  by  burning  a  mixture  of  clay  and 
straw  or  sawdust.  The  combustible  straw  or  sawdust 
burns  out  leaving  the  material  light,  porous,  and  tough. 
Nails  and  screws  may  be  driven  into  porous  terra-cotta, 
and  a  wood  saw  can  be  used  to  cut  it.  Fig.  74  shows 
typical  forms  of  terra-cotta  units  used  for  building  blocks, 
for  fireproofing  and  also  shows  typical  forms  of  terra-cotta 
lumber. 

Drain  Tile  and  Sewer  Pipe.  —  Drain  tile  is  made  from 
carefully  selected  clay.  The  clay  is  burned  at  compara- 
tively low  temperatures,  and  the  resulting  material  is 
porous  so  that  soil  moisture  will  readily  pass  through  the 
walls  of  the  drain  tile.  Drain  tile  are  usually  molded  by 
forcing  through  a  die  a  tube  of  clay,  and  cutting  off  suitable 
lengths  with  wire. 

Sewer  pipe  is  burned  at  such  high  temperatures  that  the 
surface  of  the  pipe  is  slightly  vitreous,  and  in  addition  a 


192 


MATERIALS  OF  ENGINEERING 


waterproof  glaze  is  given  to  the  surface  by  the  addition 
of  salt  during  the  process  of  burning.  If  made  without 
socket  ends,  sewer  pipe  is  molded  as  is  drain  tile;  for 
pieces  of  sewer  pipe  with  socket  ends  a  separate  mold  is 


Terra  Cotta    Blocks  -Tor  Walls,  Piers  &  Fireproof! no. 
(These  are  made  in  many  different  sizes 
and  shapes) 

Floor 


Terra  Cot-ra  Blocks  used  for  Arch  Supporting  Floor 


.--Plaster 

Terra  Coffa 
Tile 


Terra  Cotta  Blocks  used 
for    Fire  proof  ing 

FIG.  74. — Typical  forms  and  uses  of  terra-cotta  blocks. 


used  for  each  piece.  Sewer  pipe  is  not  porous  and  is  used 
for  locations  where  it  is  not  desired  to  have  the  pipe  absorb 
water  from  the  surrounding  soil.  The  joints  between  dif- 
ferent pieces  in  a  line  of  sewer  pipe  are  made  tight  by  the 
use  of  Portland-cement  mortar. 


STONE,  BRICK  AND  TERRA-COTTA  193 

Strength  of  Porcelain  and  Stoneware. — The  strength 
of  porcelain  and  stoneware,  both  special  burnt  clay  prod- 
ucts, is  of  importance,  especially  in  connection  with  the 
design  and  use  of  strain-insulators — insulators  for  electric 
transmission  lines,  which  have  to  carry  the  load  brought 
on  them  by  the  weight  of  spans  of  wire.  Test  data  for 
the  strength  of  porcelain  and  stoneware  are  very  few. 
The  results  of  a  series  of  tests  by  Prof.  J.  E.  Boyd  at 
the  Ohio  State  University  indicate  that  for  porcelain  the 
ultimate  tensile  strength  is  not  less  than  3000  Ib.  per  sq.  in., 
and  for  stoneware,  from  1100  to  2200  Ib.  per  sq.  in.:  the 
ultimate  compressive  strength  for  porcelain  and  for  high 
grade  stoneware  is  about  20,000  Ib.  per  sq.  in.,  with  a  pro- 
portional limit  at  about  7000  Ib.  per  sq  in.:  the  modulus 
of  elasticity  for  both  tension  and  compression  is  about 
10,000,000  Ib.  per  sq.  in.  for  porcelain,  and  about  7,500,000 
Ib.  per  sq.  in.  for  stoneware. 

Sand-lime  Brick. — Sand-lime  bricks  are  not  burnt-clay 
products,  but  as  they  are  made  of  the  same  standard  size 
as  are  clay  building  brick,  and  as  their  uses  are  the  same  as 
those  of  building  brick,  they  will  be  mentioned  here.  Sand- 
lime  bricks  are  made  of  a  finely  ground  mixture  of  slaked 
lime  and  sand.  The  materials,  thoroughly  mixed,  are 
pressed  into  shape  in  molds,  after  which  they  have  suffi- 
cient stiffness  to  hold  their  form  under  their  own  weight. 
The  molded  bricks  are  carried  on  small  cars  into  a  long 
cylinder  where  they  are  subjected  to  steam  at  a  pressure 
of  about  120  Ib.  per  square  inch  for  a  period  of  about  10  hr. 
The  action  of  the  steam  causes  cementing  action  between 
the  lime  and  the  sand,  and  when  the  sand-lime  bricks  are 
taken  from  the  steam  cylinder  they  have  a  fairly  high 
strength.  The  strength  of  sand-lime  bricks  increases  for 
some  months  after  they  are  made.  Sand-lime  bricks  are 
used  for  general  building  purposes.  The  strength  of  sand- 
lime  brick  is  about  three-quarters  that  of  ordinary  clay 
building  brick. 

Strength  of  Brick  and  Terra -cotta  and  of  Brick  Masonry 
and  Terra -cotta  Masonry. — As  in  the  case  of  stone  masonry 

13 


194  MATERIALS  OF  ENGINEERING 

brick  masonry  is  always  used  in  compression.  The  com- 
pressive strength  per  square  inch  of  brick  masonry  is  much 
less  than  the  compressive  strength  of  the  individual  bricks. 
In  general,  strong  bricks  make  stronger  masonry  than  do 
weak  bricks,  but  the  quality  of  mortar  used,  the  closeness 
of  fit  between  adjacent  bricks,  and  the  care  used  in  laying 
the  brick,  all  are  important  factors  in  the  strength  of  brick 
masonry.  In  walls,  foundations,  and  pavements  cracks 
occur  most  commonly  along  the  mortar  joints.  When 
individual  bricks  break  they  nearly  always  crack  across  by 
flexure  rather  than  crushing  by  compression,  and  the 
flexural  strength  of  individual  bricks  is  a  better  general 
index  of  their  quality  than  is  the  compressive  strength. 

Tests  of  compressive  strength  of  brick  masonry  have  been 
made  at  various  testing  laboratories.  Table  13  gives 
the  summarized  results  of  a  number  of  such  tests.  Table  4 
gives  allowable  working  loads  on  brick  masonry. 

Table  13  gives  the  summarized  results  of  two  series 
of  tests  of  terra-cotta  block  piers  made  at  the  University 
of  Illinois. 

Durability  of  Brick  and  of  Terra-cotta  Masonry. — Brick 
masonry  and  terra-cotta  masonry  if  well  made  from  good 
materials  are  as  nearly  permanent  as  any  structural 
material.  However,  they  may  finally  suffer  disintegra- 
tion under  weathering,  and  usually  the  freezing  and 
consequent  expansion  of  absorbed  water  is  a  prominent 
factor  in  the  disintegrating  process.  Porous  brick  absorbs 
much  more  water  than  does  hard-burned  brick,  and  porous 
brick,  in  general,  weathers  poorly.  Lime  and  other  salts 
are  dissolved  out  of  brick  masonry  which  is  exposed  to  the 
weather  and  sometimes  streaks  of  lime  are  deposited  on 
the  surface  of  the  masonry  as  the  result  of  this  leaching-out 
action.  Sand-lime  brick,  which  when  new  are  nearly  white, 
and  other  light-colored  brick  are  particularly  liable  to  show 
disfigurement  by  the  leaching  out  of  salts. 

The  mortar  joints  in  either  stone  or  brick  masonry  are 
especially  liable  to  damage  by  weathering.  Pointing 
masonry  consists  in  filling  the  edges  of  the  joints  to  the 


STONE,  BRICK  AND  TERRA-COTTA 


195 


depth  of  about  an  inch  with  rich  mortar  packed  in  as  com- 
pactly as  possible.  This  pointing  offers  resistance  to  the 
action  of  the  weather  at  the  joints,  and  increases  the  en- 
durance of  the  masonry. 

TABLE  13. — STRENGTH  IN  COMPRESSION  OF  BRICK  PIERS  AND  OF  TERRA- 
COTTA BLOCK  PIERS 

The  values  given  are  based  on  test  data  from  Watertown  Arsenal,  Cornel 
University,  U.  S.  Bureau  of  Standards  (Pittsburgh  Laboratory)  and  the 
University  of  Illinois. 

The  weight  of  masonry  may  be  taken  as  about  5  Ib.  per  cu.  ft.  less  than 
the  weight  of  the  stone  or  brick  used. 


Brick  or  block  used 

Mortar 

Ultimate  in 
compression, 
Ib.  per  sq.  in. 

Vitrified  brick  
Pressed  (face)  brick  .  .  . 
Pressed  (face)  brick  .  .  . 
Common  brick  

part  Portland  cement,  3  parts  sand 
part  Portland  cement,  3  parts  sand 
part  lime,  3  parts  sand 
part  Portland  cement,  3  parts  sand 

2,800 
2,000 
1,400 
1,000 

Common  brick  .  . 

part  lime,  3  parts  sand 

700 

Terra-cotta  block  

part  Portland  cement,  3  parts  sand 

3,000 

Test  data  for  piers  built  of  sand-lime  brick  are  lacking,  but,  judging 
from  test  data  for  individual  brick,  sand-lime  brick  piers  might  be  expected 
to  be  about  three-quarters  as  strong  as  piers  built  of  common  brick. 


Selected  References  for  Further  Study 

BAKER:  "A  Treatise  on  Masonry  Construction,"  New  York,  1909.  A 
standard  treatise  by  an  American  civil  engineer. 

RIES:  ''Building  Stones  and  Clay  Products,"  New  York,  1912. 

BLEININGER:  Clay  Products  as  an  Engineering  Material,  paper  read  before 
the  International  Engineering  Congress  at  San  Francisco  in  1915.  Re- 
printed in  part  in  the  Clay  worker  for  December,  1915,  and  January, 
1916. 

PARR  AND  ERNEST:  A  Study  of  Sand-lime  Brick,  Illinois  State  Geological 
Survey,  Bulletin  18. 

TALBOT  AND  ABRAMS:  Tests  of  Brick  Columns  and  Terra-Cotta  Columns, 
University  of  Illinois,  Engineering  Experiment  Station  Bulletin  27. 

BOYD:  Elasticity  and  Strength  of  Stoneware  and  Porcelain,  Journal  of 
A.  S.  M.  E.,  Mar.-h,  1916. 


CHAPTER  XVII 

CEMENTING  MATERIALS :  GYPSUM,  LIME,   NATURAL 
CEMENT,  AND  PORTLAND  CEMENT 

Cementing  Materials. — A  number  of  substances  possess 
the  property  when  mixed  to  a  paste  with  water  of  harden- 
ing into  a  solid  under  the  chemical  and  crystallizing  actions 
set  up  in  the  paste.  Such  substances  are  very  valuable  to 
the  structural  engineer.  Walls,  foundations,  piers,  and 
other  structural  units  may  be  constructed  by  filling  molds 
with  the  paste  and  allowing  it  to  harden  into  a  solid  of  the 
desired  shape,  or  the  paste  may  be  used  as  a  binding 
material  for  the  units  of  brick  or  of  stone  masonry.  The 
principal  cementing  materials  used  in  structural  work  are 
gypsum,  lime  (including  quicklime,  hydrated  lime,  and 
hydraulic  lime),  natural  cement,  and  Portland  cement. 

Gypsum.— ^-Gypsum  is  a  combination  of  sulphate  of  lime 
with  water  of  crystallization  (CaSO4  +  2H2O).  Large 
deposits  of  impure  gypsum  rock  are  found  in  various  lo- 
calities in  the  United  States.  If  gypsum  rock  is  subjected 
to  a  temperature  exceeding  212°F.  a  portion  of  the  water  of 
crystallization  is  driven  off,  and  the  solid  residue  left,  when 
finely  ground,  is  capable  of  reabsorbing  water  and  harden- 
ing into  a  solid  mass.  The  nature  of  the  product  depends 
on  the  purity  of  the  raw  materials,  upon  the  temperature 
used  in  driving  off  the  water  of  crystallization,  and  upon 
the  addition  of  foreign  ingredients  to  retard  or  accelerate 
the  set.  The  products  of  the  calcination  of  gypsum  rock 
are  marketed  under  a  variety  of  names,  such  as  plaster  of 
Paris,  dental  plaster,  hard  wall  plaster,  Keene's  cement, 
and  gypsum  plaster. 

Manufacture  of  Gypsum  Products. — The  general  process 
of  preparing  gypsum  products  as  used  in  the  United  States 
consists  in:  (1)  grinding  gypsum  rock,  which  is  the  raw 

196 


CEMENTING  MATERIALS 


197 


material  most  commonly  used;  (2)  calcining  a  charge  of 
ground  gypsum  rock  at  a  temperature  varying  from  270° 
to  400°F.;  (3)  fine  grinding  of  the  calcined  product;  (4) 
for  some  gypsum  products  the  addition  of  substances  which 
retard  the  setting  of  the  calcined  powder  when  mixed  with 
water.  Gypsum  plaster  to  be  used  for  wall  finish  is  rend- 
ered more  plastic  by  the  addition  of  clay  or  of  hydrated 
lime.  The  cohesive  ness  of  such  plaster  is  increased  by 
adding  hair  or  shredded  wood  fiber. 


•Wire  Mesh 


SPLIT          HOLLOW  HOLLOW  ROOF  TILE 

Gypsum    Blocks  for  Walls  and    Roofs 


^,-Wire  Mesh 


i 

*        —  —  \ 

4'+o  10' 


Long   Span    Gypsum    Roof  Tile 
(To  be  carried  directly  on  Roof  Truss) 

FIG.  75. — Typical  forms  of  gypsum  blocks  and  gypsum  tile. 

Structural  Uses  of  Gypsum  Products. — As  a  structural 
material  gypsum  plaster  is  very  light,  is  a  good  fire  resist- 
ant, is  inexpensive,  and  possesses  a  fair  degree  of  compres- 
sive  strength.  Its  general  use  is  for  structural  members 
in  which  lightness  or  fire-resisting  qualities  are  of  prime 
importance.  Gypsum  plasters  are  widely  used  for  wall 
finish.  Gypsum  blocks  are  used  for  building  curtain  walls 
in  buildings  (curtain  walls  are  those  carrying  no  load  from 


198  MATERIALS  OF  ENGINEERING 

floors  above  them),  for  roof  slabs,  and  for  fire  proofing 
around  columns.  Typical  forms  of  gypsum  blocks  and 
tile  are  shown  in  Fig.  75.  Structural  gypsum  weighs  not 
more  than  80  Ib.  per  cubic  foot.  The  lightness  of  gypsum 
makes  it  possible  for  workmen  to  handle  large-sized  blocks, 
which  makes  the  work  of  wall-building  or  of  roof-laying 
quicker.  For  roof  slabs  and  floor  slabs  a  mixture  of  gyp- 
sum and  wood  fiber  reinforced  with  steel  wire  is  used. 
Gypsum  mortar  is  used  for  the  binding  of  gypsum  fireproof- 
ing,  and  for  hard  wall  finish. 

Gypsum  as  a  Fireproofing  Material. — Gypsum  makes 
a  good  fireproofing  material  for  steel  structures.  The 
strength  of  gypsum  is  destroyed  by  long-continued  heat 
when  the  water  of  crystallization  is  finally  driven  off,  but 
a  great  deal  of  heat  is  required  to  evaporate  the  water  of 
crystallization,  and  as  the  evaporation  proceeds  the  gyp- 
sum does  not  crack  or  spall,  but  its  surface  is  converted 
into  an  anhydrous  powder  which  acts  as  an  excellent  heat 
insulator,  retarding  the  further  evaporation  of  water  of 
crystallization  of  the  inside  layers  of  gypsum.  Gypsum 
blocks  are  used  for  fireproofing  in  much  the  same  manner 
as  is  shown  in  Fig.  74  for  terra-cotta  blocks. 

Strength  of  Structural  Gypsum. — The  ultimate  com- 
pressive  strength  of  test  cylinders  of  gypsum  has  been 
found  to  vary  all  the  way  from  70  Ib.  per  square  inch  up  to 
3,000  Ib.  per  square  inch  depending  upon  the  amount  of 
water  used  in  mixing  the  gypsum  paste,  the  completeness 
of  drying  out  of  water  after  the  gypsum  paste  had  set,  the 
amount  of  foreign  ingredients  mixed  with  the  gypsum  to 
retard  its  rate  of  setting,  and  the  temperature  used  in 
calcining  the  gypsum  rock.  For  highest  strength  the 
least  possible  amount  of  water  should  be  used  in  mixing 
the  gypsum  paste.  From  33  to  38  per  cent,  of  water  is 
necessary  to  make  gypsum  paste  sufficiently  plastic  to  fill 
molds  properly,  and  this  percentage  of  water  is  more  than 
sufficient  to  hydrate  the  gypsum.  For  gypsum  from  the 
same  source  of  raw  material  uniformity  of  consistency  of 
gypsum  paste  insures  a  good  degree  of  uniformity  of  strength. 


CEMENTING  MATERIALS  199 

With  care  in  mixing  and  drying  out,  gypsum  can  be  pro- 
duced regularly  with  a  compressive  strength  of  1,400  Ib. 
per  square  inch.  However,  when  practicable  tests  should 
be  made  to  determine  the  strength  of  any  lot  of  gypsum 
blocks  or  other  structural  members. 

Gypsum  gains  its  full  strength  in  a  few  hours  if  carefully 
kiln-dried,  and  air-cured  gypsum  gains  strength  so  rapidly 
that  forms  may  be  removed  the  day  after  the  gypsum  is 
poured.  Gypsum  is  weakened  by  prolonged  exposure  to 
water,  and  should  not  be  used  where  it  will  be  kept  as  oak 
for  considerable  periods  of  time. 

The  modulus  of  elasticity  for  structural  gypsum  is  about 
1,000,000  Ib.  per  square  inch.  The  stress-strain  diagram 
for  gypsum  is  very  nearly  a  straight  line  up  to  the  ultimate. 

Lime. — The  basis  of  the  cementing  action  of  lime  mortar, 
natural  cement,  and  Portland  cement  is  the  absorption  of 
water  and  subsequent  hardening  of  calcium  oxide  or  cal- 
cium hydroxide.  Calcium  oxide  (CaO)  is  known  as 
quicklime.  It  is  prepared  by  heating  limestone  which  is 
mainly  calcium  carbonate  (CaCO3).  Under  the  influence 
of  heat  carbon  dioxide  is  driven  off  leaving  calcium  oxide  or 
quicklime.  The  heating  or  " burning' '  of  lime  is  carried 
on  in  a  brick-lined  stack  known  as  a  lime  kiln.  Fuel, 
usually  bituminous  coal,  is  burned  on  grates  at  the  side  of 
the  stack;  limestone  is  fed  into  the  top,  the  hot  gases  pass 
through  the  limestone,  and  the  quicklime  which  is  produced 
is  removed  at  the  bottom.  In  another  type  of  kiln  alter- 
nate layers  of  limestone  and  coal  are  fed  into  the  top  of  the 
stack.  Quicklime  must  not  be  left  exposed  to  the  air.  If 
it  is  so  exposed  it  absorbs  carbon  dioxide  and  is  then  trans- 
formed to  powdered  calcium  carbonate  (air-slacked  lime) 
and  is  useless  for  cementing  purposes. 

Hydrated  Lime. — If  quicklime  is  mixed  with  about  one- 
third  its  weight  of  water  it  is  changed  from  calcium  oxide 
to  calcium  hydroxide  (Ca(OH)2).  This  change  is  accom- 
panied by  the  evolution  of  considerable  heat  and  by  a  very 
great  increase  in  volume.  The  product  is  a  fine  white 
powder,  which  when  mixed  with  more  water  absorbs  water 


200  MATERIALS  OF  ENGINEERING 

of  crystallization  and  hardens.  If  it  is  attempted  to  use 
lime  as  a  mortar  unmixed  with  other  substances  the  great 
shrinkage  which  takes  place  while  the  hardening  process 
goes  on  causes  wide  cracks  in  the  hardened  mass.  A  mix- 
ture of  1  part  lime  and  2  parts  sand  is  commonly  used  to 
make  lime  mortar. 

Two  methods  of  making  lime  mortar  are  in  use:  in  one, 
quicklime  is  brought  to  the  place  where  mortar  is  to  be 
used,  and  is  mixed  with  water,  or  " slaked"  on  the  job  by 
the  workman;  in  the  other  method  the  quicklime  is  slaked 
at  the  kiln  under  expert  supervision,  and  the  amount  of 
water  necessary  for  complete  slaking,  carefully  computed, 
is  added.  The  slaked  lime  is  ground  fine,  screened  through 
a  fine  sieve,  and  packed  in  bags.  On  the  job  this  hy- 
drated  lime  is  mixed  with  sand  and  water  to  form  mortar. 

Lime  mortar  will  not  harden  under  water  nor  in  any  place 
unless  air  has  free  access  to  it.  Its  principal  uses  are  for 
the  binding  material  for  brick  masonry  and  stone  masonry, 
and  for  plastering  inteiior  walls. 

Natural  Cement. — During  the  construction  of  the  Eddy- 
stone  lighthouse  in  England  the  engineer  in  charge,  John 
Smeaton,  discovered  that  by  burning  a  limestone  contain- 
ing some  clay  there  was  produced  a  lime  which  would 
harden  under  water.  This  product  known  as  "  hydraulic  " 
lime  is  still  widely  used  in  Europe.  In  America  there  have 
bejen  discovered  large  deposits  of  argillaceous  (clay-bearing) 
limestone  which,  when  heated  to  about  2,000°F.  give  off 
carbon  dioxide,  leaving  a  clinker.  This  clinker  is  known  as 
" natural"  cement.  It  will  not  slake  in  air,  and  when 
mixed  with  water  it  hardens  either  in  air  or  under  water. 
Natural  cement  varies  greatly  in  quality  depending  on  the 
clay  content  of  the  limestone  deposit.  It  is  cheap,  and  is 
used  to  a  limited  extent  for  mortar  for  masonry  and  for 
concrete  work  where  strength  is  not  a  prime  requisite. 
Its  use  is  decreasing  in  this  country;  Portland  cement  is 
now  generally  used  rather  than  natural  cement. 

Puzzolan  Cement  and  Slag  Cement. — In  some  countries 
where  there  are  deposits  of  volcanic  ash  a  cement  is  made 


CEMENTING  MATERIALS  201 

by  grinding  up  clayey  material  from  this  ash  and  mixing 
it  with  hydrated  lime,  giving  a  hydraulic  cement.  As  such 
deposits  of  volcanic  material  differ  greatly  in  quality  the 
resulting  cement  is  a  rather  variable  material.  Puzzolan 
cement  is  of  historic  importance,  being  widely  used  in  the 
days  of  the  Roman  empire,  but  except  for  occasional  local 
use  near  deposits  of  suitable  volcanic  ash  it  is  not  much 
used  today. 

A  cement  in  which  ground  blast  furnace  basic  slag  is 
mixed  with  hydrated  lime  was  formerly  quite  commonly 
used.  Portland  cement  has  almost  entirely  displaced  it. 
Slag  cement  should  be  carefully  distinguished  from  Portland 
cement  made  with  blast  furnace  slag  as  an  ingredient. 

Portland  Cement. — To  the  engineer  the  most  important 
of  the  cementing  materials  is  Portland  cement.  This  is  an 
artificial  mixture  of  lime-bearing  material  with  clayey 
material.  The  mixture  is  burned  to  a  clinker  at  a  tempera- 
ture of  incipient  fusion  and  afterward  ground  to  a  fine 
powder.  Portland  cement  does  not  deteriorate  to  any 
appreciable  extent  in  dry  air,  it  hardens  in  air  or  under 
water,  and  in  hardening  Portland-cement  mortar  shrinks 
much  less  than  do  other  kinds  of  mortar. 

Raw  Materials  for  Portland  Cement. — The  lime-bear- 
ing material  used  for  Portland-cement  manufacture  is  some 
form  of  calcium  carbonate  (CaCO3);  limestone,  marl,  or 
chalk  are  the  materials  commonly  used.  The  clayey  ma- 
terials include  clay,  shale,  and  blast-furnace  slag  (which  con- 
tains some  calcium  carbonate  also).  In  the  Lehigh  Valley 
region  Portland  cement  is  produced  from  cement  rock, 
which  is  a  natural  mixture  of  limestone  and  shale  in  nearly 
the  right  proportions  for  making  Portland  cement.  In  the 
Illinois  River  region  there  are  found  alternate  layers  of 
limestone  and  shale.  In  the  Ohio  region  great  beds  of  marl 
furnish  the  lime-bearing  ingredient,  and  in  the  Dakota 
region  chalk  is  used.  Near  the  great  pig-iron  centers  blast- 
furnace slag  furnishes  the  clayey  ingredient  and  some  of  the 
lime,  and  a  very 'pure  limestone  is  used  to  furnish  the  re- 
mainder of  the  lime-bearing  ingredient. 


202  MATERIALS  OF  ENGINEERING 

Manufacture  of  Portland  Cement. — Fig.  76  shows  in  dia- 
gram the  process  of  making  Portland  cement.  The  lime- 
bearing  ingredient  and  the  clayey  ingredient  for  making 
Portland  cement  are  first  crushed  to  pebble  size  (unless  the 
raw  material  is  found  in  a  finely  divided  form).  After  the 
crushing  the  material  is  driedr  in  horizontal  rotary  driers. 
The  ingredients  are  then  ground  finer  in  rotating  cylinders 
containing  steel  balls,  which  are  known  as  "ball  mills/' 
and  then  are  mixed  in  the  proper  proportion,  which  is  deter- 
mined by  chemical  analysis.  After  the  mixing  a  third 
grinding  takes  place.  This  is  carried  on  in  a  rotating  tube 
mill  filled  with  flint  pebbles,  and  the  material  is  reduced  to 
a  fine  powder.  The  fine  powder  is  then  carried  to  hoppers 
from  which  it  is  fed  into  rotary  kilns  about  120  ft.  long. 
These  kilns  make  about  one  revolution  per  minute,  and  are 
heated  by  a  burning  blast  of  powdered  coal.  They  are 
slightly  inclined,  and  the  material  which  is  fed  gradually 
travels  from  one  end  of  the  kiln  to  the  other,  and  is  heated 
to  incipient  fusion  at  a  temperature  of  about  2,700°F.  At 
the  discharge  end  of  the  kiln  the  clay  and  lime  have  been  / 
burned  to  a  hard  lumpy  clinker.  This  clinker  is  carried  to 
storage  bins  where  it  is  cured  for  about  10  days,  after  which 
it  is  crushed  in  a  rock  crusher  and  then  ground  into  small 
pieces,  in  some  form  of  coarse-grinding  mill  ("  preliminary 
mills"  in  Fig.  76).  At  this!  stage  of  the  process  a  carefully 
calculated  amount  of  gypsum  is  added  to  retard  the  rapidity 
of  setting  of  the  finished  product.  The  final  step  in  the 
production  of  Portland  cement  is  the  grinding  of  the  ingre- 
dients in  tube  mills  to  a  very  fine  powder. 

The  above  process  of  Portland-cement,  manufacture  is 
the  one  most  commonly  used  in  this  country,  and  is  known 
as  the  dry  process.  When  marl  is  used  as  the  lime-bearing 
ingredient  the  clay  and  the  marl  are  mixed  wet,  and  the 
ingredients  are  pumped  into  the  kiln,  drying  of  the  wet 
mixture  taking  place  in  the  first  part  of  the  progress  through 
the  kiln.  This  process  is  known  as  the  wet  process. 

Portland  cement  is  usually  packed  for  shipment  in 
cloth  bags,  each  containing  1  cu.  ft.  of  cement,  which 


CEMENTING  MATERIALS 


203 


204  MATERIALS  OF  ENGINEERING 

weighs  about  94  Ib.     Four  bags  of  cement  are  equivalent 
to  1  bbl. 

Selected  References  for  Further  Study 

MILLS:  "The  Materials  of  Construction,"  New  York,  1915,  Chaps.  I-VI 

inclusive. 

ECKEL:  "Cements,  Limes,  and  Plasters,"  New  York,  1905. 
TAYLOR  AND  THOMPSON:  "Concrete,  Plain  and  Reinforced,"  New  York, 

1916,  Chap.  XXXI. 


CHAPTER  XVIII 
CONCRETE 

BY    H.    F.    GONNERMAN 

Portland  Cement  Concrete. — Concrete  consists  of  a 
mixture  of  cement,  water  and  non-cementing,  or  inert 
materials,  such  as  sand  and  gravel.  Portland  cement, 
because  of  its  uniformity,  reliability,  and  strength  is  gener- 
ally used  as  the  cementing  material  in  making  concrete. 
Natural  and  puzzolan  cements  are  used  to  a  very  limited 
extent  and  then  only  in  special  kinds  of  work.  If  Portland 
cement  unmixed  with  other  solid  materials  were  used  in 
making  buildings,  bridges,  and  other  structure's,  such  con- 
struction would  be  very  costly  and  moreover  the  cement 
after  hardening  might  show  a  tendency  to  crack  badly. 
In  order,  therefore,  to  produce  an  economical  and  satis- 
factory building  material  the  Portland  cement  is  always 
combined  with  a  relatively  large  proportion  of  inert, 
solid  material  called  aggregate. 

Plain  Concrete  and  Reinforced  Concrete. — Concrete 
is  a  brittle  material  and  like  nearly  all  other  brittle  materials 
is  much  stronger  in  compression  than  in  tension.  Its 
tensile  strength  is  so  low  that  it  is  usually  neglected  in 
making  computations  of  the  strength  of  concrete  struc- 
tures. Concrete  construction  in  which  concrete  alone 
is  used  is  known  as  plain  concrete.  Plain  concrete  is  used 
for  massive  construction  work  or  for  parts  of  structures 
carrying  only  compressive  load.  Heavy  foundations,  mas- 
sive dams,  piers,  heavy  walls,  massive  arches,  sidewalks, 
and  narrow  pavements  furnish  examples,  of  plain  concrete 
construction. 

205 


206  MATERIALS  OF  ENGINEERING 

In  concrete  structural  members  in  which  there  exists 
tensile  stress  the  strength  to  resist  such  stress  is  furnished 
by  embedding  steel  bars  in  the  concrete.  Such  concrete 
construction  is  known  as  reinforced  concrete.  Some  of  the 
principal  examples  of  the  uses  of  reinforced  concrete  are: 
beams,  columns,  footings,  fk>or  slabs,  roofs,  bridges,  reser- 
voirs, culverts,  chimneys,  light  arches,  and  retaining  walls. 
Reinforcing  steel  is  also  used  in  concrete  structures  such 
as  wide'  pavements  in  which  stresses  due  to  temperature 
changes  would  cause  injurious  cracks  in  plain  concrete. 
The  somewhat  complex  mechanics  of  reinforced-concrete 
structures  will  not  be  taken  up  in  this  book. 

Concrete  Aggregates. — The  quality  of  the  aggregates 
used  in  making  concrete  is  of  very  great  importance.  There 
is  as  much  likelihood  of  poor  concrete  resulting  from  poor 
aggregates  as  from  poor  cement.  Where  it  can  be  done  the 
aggregates  to  be  used  should  be  tested  by  making  sample 
test  pieces  of  concrete,  using  the  aggregates  and  a  good 
grade  of  cement,  and  comparing  the  strength  of  these  test 
pieces  with  that  of  other  test  pieces  made  up  with  cement 
from  the  same  lot  and  with  aggregate  known  to  be  of  good 
quality. 

In  general,  the  qualities  which  are  desired  in  a  material 
to  be  used  as  aggregate  in  making  concrete  are  :  cleanness, 
hardness,  durability,  toughness  and  structural  strength. 
The  material  should  also  have  proper  gradation  of  size  of 
particles.  Among  the  materials  commonly  used  as  aggre- 
gate in  making  concrete  are:  sand,  gravel,  broken  stone, 
limestone  screenings  and  cinders.  Blast  furnace  slag  has 
also  been  used  for  the  aggregate  in  concrete  with  satis- 
factory results.  For  special  purposes  other  materials 
than  those  just  mentioned  are  sometimes  used;  for  example, 
in  the  construction  of  concrete  ships  a  lightweight  aggre- 
gate consisting  of  burned  clay  has  been  used.  By  using 
this  material  a  strong  concrete  approximately  three-fourths 
as  heavy  as  the  ordinary  stone  or  gravel  concrete  was 
obtained  thus  reducing  materially  the  dead  weight  of 
the  vessel  and  thereby  increasing  its  carrying  capacity. 


CONCRETE  207 

Aggregates  are  commonly  classed  as  fine  aggregates,  and 
coarse  aggregates  depending  on  the  maximum  size  of  the 
particles  composing  them.  Material  having  particles 
ranging  in  size  from  the  very  smallest  dust  particles  to 
particles  about  K  in.  in  diameter  is  called  fine  aggregate. 
Sand,  gravel  screenings,  and  stone  screenings  are  generally 
used  for  the  fine  aggregate  in  concrete.  Aggregates  having 
particles  ranging  in  size  from  y±  in.  in  diameter  to  1^  in." 
or  more  are  called  coarse  aggregates.  Gravel  and  broken 
stone  are  used  in  large  quantities  for  coarse  aggregate. 

In  order  to  produce  a  dense,  economical  and  strong 
concrete  it  is  always  necessary  to  use  a  mixture  of  fine  and 
coarse  aggregate.  A  mixture  of  fine  aggregate  (generally 
sand),  cement  and  water  is  known  as  mortar. 

To  obtain  the  best  results  it  is  essential  that  both  fine 
and  coarse  aggregate  be  clean  and  well  graded.  An  aggre- 
gate is  said  to  be  well  graded  when  it  contains  particles 
or  grains  ranging  in  size  from  the  finest  to  the  largest,  or 
coarsest,  with  the  coarser  particles  predominating.  When 
stone  screenings  are  used  for  fine  aggregate  they  should  be 
free  from  dust  and  the  particles  should  be  of  angular  shape. 
Particles  which  are  thin,  flat  and  elongated  are  easily  brok- 
en. Careful  tests  have  shown  that  sands  having  rounded 
grains  are  as  suitable  for  fine  aggregate  in  mortar  and  con- 
crete as  are  sands  having  sharp  angular  particles. 

Concrete  is  frequently  given  a  special  name  from  the 
kind  of  aggregate  used;  thus  we  have  broken-stone  con- 
crete, gravel  concrete,  cinder  concrete,  blast-furnace  slag 
concrete  and  rubble  or  cyclopean  concrete  (concrete  in 
which  part  of  the  aggregate  consists  of  pieces  of  unbroken 
rock,  sometimes  several  feet  in  diameter). 

Undesirable  Ingredients  in  Concrete  Aggregates. — As 
previously  stated  only  clean  aggregates  should  be  used  if 
the  best  results  are  to  be  obtained.  In  general,  it  may  be 
said  that  most  of  the  trouble  experienced  with  faulty  mor- 
tar or  concrete  is  due  to  the  presence  of  undesirable  ingre- 
dients in  the  fine  aggregate  used  which  is  ordinarily  a  sand. 
The  effect  of  impurities  in  the  sand  on  the  strength  and 


208  MATERIALS  OF  ENGINEERING 

other  properties  of  mortar  and  concrete  are  generally 
dependent  upon  the  character  of  the  impurities,  the  rich- 
ness of  the  mixture  and  the  grading  of  the  sand.  Clay, 
silt  and  loam  are  usually  considered  undesirable  ingredients 
in  concrete  aggregate  and  when  present  in  any  considerable 
amount  they  should  be  removed  by  washing  the  aggregate 
before  it  is  mixed  with  the  cement.  Some  experimenters 
have  found  that  lean  mortars  may  be  helped  by  the  addi- 
tion of  small  quantities  of  fine,  pure  clay  as  it  increases  the 
smoothness  of  the  mixture  and  helps  to  fill  voids  thus 
increasing  the  density  and  water-tightness  of  the  mortar. 
In  rich  mixtures,  however,  it  may  have  a  harmful  influence 
since  in  these  mixtures  the  cement  furnishes  all  the  fine 
material  necessary  for  high  density.  Aggregate  contain- 
ing lumps  of  clay  should  never  be  used  for  mortar  or 
concrete. 

Organic  material,  like  vegetable  loam,  is  a  very  unde- 
sirable ingredient  in  concrete  sands  even  when  present  in 
small  quantities.  Organic  material  in  many  instances  has 
been  found  to  retard  or  prevent  the  hardening  of  mortar 
and  concrete  and  to  reduce  greatly  the  strength  of  concretes 
and  mortars. 1  Sand  suspected  of  containing  organic  matter 
should  be  carefully  tested  before  being  used. 

Mica  is  an  injurious  ingredient  in  sand  or  broken  stone 
used  for  concrete  and  only  aggregates  free  from  mica  should 
be  used.  If  it  is  necessary  on  account  of  local  conditions 
to  use  aggregate  containing  undesirable  ingredients  the 

1  A  simple  test  for  detecting  the  presence  of  organic  impurities  in  sands 
which  may  be  used  in  the  laboratory  or  in  the  field  has  been  developed  by 
D.  A.  Abrams  and  O.  E.  Harder.  The  test  is  known  as  the  "colorimetric 
test"  and  is  carried  out  in  the  field  as  follows:  "Fill  a  12-oz.  graduated 
prescription  bottle  to  the  4^-oz.  mark  with  the  sand  to  be  tested.  Add  a 
3  per  cent,  solution  of  sodium  hydroxide  (NaOH)  until  the  volume  of  the 
sand  and  solution,  after  shaking,  amounts  to  7  oz.  Shake  thoroughly  and 
let  stand  for  24  hours.  Observe  the  color  of  the  clear  liquid  above  the  sand. 
If  the  solution  resulting  from  this  treatment  is  colorless,  or  has  a  light 
yellowish  color,  the  sand  may  be  considered  satisfactory  in  so  far  as  organic 
impurities  are  concerned.  On  the  other  hand,  if  a  solution  which  is  dark 
red  or  black  in  color  is  produced,  the  sand  should  be  rejected  or  used  only 
alter  it  has  been  subjected  to  tests  for  strength  in  a  mortar  or  concrete." 


CONCRETE  209 

concrete  should  be  made  richer  in  cement  than  would  be 
necessary  for  clean,  well-graded  aggregate. 

Proportioning  Aggregate  and  Cement  for  Concrete. — The 
problem  that  confronts  the  engineer  in  proportioning  ce- 
ment and  aggregate  for  concrete  is  to  produce  from  the 
materials  at  his  disposal,  a  concrete  which  shall  possess  cer- 
tain definite  physical  properties  with  the  least  expenditure 
for  materials  and  labor.  The  properties  desired  in  the 
concrete  are  generally  governed  by  the  use  to  which  the 
concrete  is  to  be  put.  In  concrete  roads  or  pavements 
resistance  to  wear  or  abrasion  is  desired.  In  buildings 
concrete  of  high  strength  is  desired,  while  in  reservoirs  and 
tanks  the  concrete  must  be  strong  and  must  also  be  im- 
permeable, or  watertight.  In  order  to  produce  a  concrete 
of  the  desired  strength  with  the  use  of  as  little  cement  as 
possible  careful  consideration  must  be  given  to  the  pro- 
portioning of  the  concrete  materials.  The  cement  is  al- 
ways the  most  expensive  ingredient  in  concrete  and  by 
properly  proportioning  the  cement  and  the  available  aggre- 
gates it  is  often  possible  to  effect  a  considerable  saving 
in  cement  and  still  produce  a  concrete  which  will  fulfill 
all  the  requirements  for  strength,  abrasion,  and  water- 
tightness.  Generally  speaking,  for  maximum  strength, 
maximum  resistance  to  passage  of  water,  maximum  resist- 
ance to  wear  and  maximum  resistance  to  disintegration 
by  such  agencies  as  acids,  alkalis,  or  electrolytic  action 
concrete  should  be  of  maximum  density.  Density1  (also 
called  "solidity  ratio")  of  concrete  means  the  ratio  of  the 
absolute  volume  of  the  solid  particles  of  cement  and  aggre- 
gate to  the  volume  of  the  resulting  concrete.  In  any  mass 
of  sand,  gravel  or  broken  stone  there  are  spaces  between 
the  individual  particles.  These  spaces  are  known  as  voids. 
In  dense  concrete  all  the  voids  are  filled  and  the  surfaces 
of  the  individual  particles  are  thoroughly  coated  with 
cement  paste.  With  different  aggregates  there  is  a  wide 

1  The  density  of  mortars  ranges  from  about  0.60  to  0.75  and  the  density 
of  concretes  from  0.70  to  0.90  depending  on  the  character  and  grading  of 
the  aggregate,  the  amount  of  cement,  and  the  amount  of  water  used. 
14 


210 


MATERIALS  OF  ENGINEERING 


variation  in  the  amount  of  cement  paste  required  to  fill  all 
the  voids  and  coat  all  the  individual  particles.  The  voids1 
in  coarse  aggregates  graded  from  Y±  to  1J^  in.  will  generally 
range  from  35  to  50  per  cent,  of  the  volume  of  the  aggregates. 
The  voids  in  the  common  sands  range  from  28  to  40  per  cent., 
depending  on  the  grading  of  the  sand  and  the  amount  of  moist- 
ure which  it  contains.  Well-graded,  dry  mixtures  of  fine  and 
coarse  aggregates  which  have  a  wide  assortment  of  indi- 
vidual particles  may  have  voids  as  low  as  12  per  cent. 
Aggregates  in  which  the  individual  particles  are  of  approxi- 
mately equal  size  have  a  much  larger  percentage  of  voids 
than  well-graded  aggregates. 

Well-graded  aggregate  in  general  makes  stronger  con- 
crete for  a  given  proportion  of  cement  than  does  an  aggre- 
gate in  which  the  coarse  and  fine  particles  are  nearly 
uniform  in  size.  It  rarely  happens  that  natural  mixtures  of 
sand  and  gravel  as  found  in  banks  and  gravel  pits  have 
such  proportions  of  fine  to  coarse  particles  as  will  make  the 
best  grade  of  concrete  unless  a  relatively  large  amount  of 
cement  is  used  (see  reference  at  end  of  chapter  to  paper  by 
R.  W.  Crum) .  On  important  work  it  is  necessary  to  make 
up  an  artificial  mixture  of  sand  and  broken  stone  or  gravel 
which  shall  be  well  graded. 

Several  methods  are  employed  by  engineers  to  deter- 
mine the  amount  of  cement  and  of  fine  and  coarse  aggre- 

1  When  the  specific  gravity  S,  and  the  weight  per  cubic  foot  W  of  the 
dry  material  are  known,  the  per  cent,  voids  P  may  be  computed  from  the 

/  W  \ 

formula  P  =  100    ( 1  —  crTZo)  '     ^ne  f°nowmg  table  which  is  taken  from 

Taylor  and  Thompson's  "Concrete,  Plain  and  Reinforced"  gives  average 
values  of  specific  gravity  for  various  concrete  aggregates: 


Material 

Specific 
gravity 

Material 

Specific 
gravity 

Sand 

2  65 

Limestone 

2  60 

Gravel            

2.66 

Trap  

2  90 

Conglomerate  

2.60 

Sandstone  

2  40 

Granite-  

2.70 

Bituminous  cinders  . 

1  50 

CONCRETE  211 

gate  to  be  used  in  order  to  produce  economically  a  concrete 
of  acceptable  quality.  The  more  important  of  these  meth- 
ods of  proportioning  will  now  be  described. 

Proportioning  by  Arbitrarily  Selected  Volumes. — This 
method  of  proportioning  is  more  widely  used  at  present 
than  any  other.  It  consists  in  mixing  arbitrarily  selected 
volumes  of  cement,  fine  aggregate  and  coarse  aggregate,  the 
exact  proportions  depending  on  the  voids  in  the  aggregate 
and  the  use  to  which  the  concrete  is  to  be  put.  In  this 
method  of  proportioning  it  is  assumed  that  a  coarse  aggre- 
gate of  good  quality  from  which  the  finest  particles  have 
been  screened  will  require,  for  a  workable  concrete  in  which 
all  the  voids  are  filled,  an  amount  of  sand  from  40  to  60  per 
cent,  of  its  volume,  or  an  average  of  50  per  cent,  under  ordi- 
nary conditions.  Although  the  cement  and  water  used  will 
give  a  volume  of  mortar  slightly  in  excess  of  the  volume  of  the 
sand  it  is  often  specified  that  one  volume  of  fine  aggregate 
to  two  volumes  of  coarse  aggregate  be  used.  The  ratio 
of  the  volume  of  cement  to  the  volume  of  the  fine  aggre- 
gate is  determined  by  the  engineer  from  experience  and 
from  a  consideration  of  the  properties  which  the  concrete 
should  possess.  Thus  we  often  see  specified  1:1^:3  (1 
volume  of  cement,  1^  volumes  of  fine  aggregate  and  3 
volumes  of  coarse  aggregate),  1:2:4,  1:2^:5,  1:3:6:  and 
1:4:8  concrete.  If  the  voids  in  the  coarse  aggregate  are 
greater  than  usual  the  amount  of  sand  is  increased  and 
proportions  such  as  1:2:3,  1:3:5,  or  1:4:6  may  be  used. 
In  case  the  voids  in  the  coarse  aggregate  are  low,  the 
amount  of  sand  is  decreased  and  proportions  such  as 
1:1^:4,  1:2:5,  and  1:3:7  are  used.  For  well-graded  ag- 
gregate common  proportions  of  cement  and  aggregate  for 
concrete  are:  for  very  rich  concrete  for  columns,  or  other 
structural  members  carrying  unusually  high  compressive 
stress,  1  part  cement,  \y%  parts  fine  aggregate  and  3  parts 
coarse  aggregate;  for  general  use  for  beams,  floor  slabs  and 
other  stress-carrying  structural  members,  1  part  cement, 
2  parts  fine  aggregate  and  4  parts  coarse  aggregate;  for 
"lean"  concrete  for  massive  work  or  filling,  1  part  cement 


212  MATERIALS  OF  ENGINEERING 

3  parts  fine  aggregate  and  6  parts  coarse  aggregate.     All 
the  above  proportions  are  by  volume. 

Proportioning  by  arbitrarily  selected  volumes  has,  in 
general,  given  good  results  when  used  by  experienced  engi- 
neers. Large  differences  in  results  may  be  obtained  if 
care  is  not  used  in  handling  and  measuring  the  materials. 
Frequently  a  larger  quantity  of  cement  than  necessary 
is  used  and  consequently  this  method  is  not  as  economical 
as  other  methods  in  which  the  materials  are  proportioned 
more  scientifically- 
Proportioning  by  Voids  in  Aggregate. — In  this  method, 
which  is  occasionally  used,  the  proportions  are  based  upon 
the  voids  in  the  fine  and  the  coarse  aggregate.  Having 
determined  the  voids  in  the  fine  and  coarse  aggregate  in 
the  condition  in  which  they  are  to  be  used  on  the  work, 
enough  fine  aggregate  is  used  to  fill  the  voids  in  the  coarse 
aggregate  and  enough  cement  to  fill  the  voids  in  the  sand. 
It  is  assumed  that  the  particles  of  cement  will  fit  into  the 
small  voids  in  the  sand  and  that  the  particles  of  sand  are 
fine  enough  to  fit  into  the  voids  in  the  coarse  aggregate 
without  increasing  the  volume  of  the  latter.  Since  it  has 
been  found  that  the  particles  of  sand  will  increase  the  vol- 
ume of  the  coarse  aggregate  by  pushing  the  particles  apart 
it  is  customary  to  add  a  volume  of  sand  from  5  to  10  per  cent, 
in  excess  of  the  voids  in  the  coarse  aggregate,  and  also 
to  use  an  amount  of  cement  slightly  in  excess  of  the  voids 
in  the  sand  in  order  to  provide  enough  paste  and  mortar 
to  fill  all  the  voids  in  the  final  mixture.  Sometimes 
the  voids  in  a  mixture  of  fine  and  coarse  aggregate  are 
determined  and  then  an  amount  of  cement  is  used  which 
is  slightly  in  excess  of  the  voids  in  the  mixed  aggregate. 
Another  method  is  to  use  a  volume  of  mortar  having  a 
fixed  ratio  of  cement  to  sand,  which  is  sufficient  to  fill  com- 
pletely the  voids  in  the  coarse  aggregate.  These  methods 
of  propoitioning  are  not  accurate  since  the  voids  in  the 
aggregate  may  vary  greatly  because  of  differences  in  com- 
pactness due  to  differences  in  methods  of  handling  the 
materials  and  also  because  the  volume  of  the  voids  in  the 


CONCRETE 


213 


fine  aggregate  is  greatly  influenced  by  the  amount  of  mois- 
ture it  contains.  Furthermore,  if  the  fine  aggregate  con- 
tains much  fine  material,  the  particles  of  cement  will 
separate  the  particles  of  the  fine  aggregate  and  since  the 
fine  aggregate  generally  contains  particles  too  coarse  to  fit 
into  the  voids  of  the  coarse  aggregate,  they  in  turn  force 
the  particles  of  the  coarse  aggregate  apart  as  shown  in 
Fig.  .77  and  increase  its  bulk.  This  separation  of  the  par- 
ticles of  the  aggregate  reduces  the  density  and  strength  of 
the  resulting  concrete. 


Courtesy  of  U.  S.  Bureau  of  Standards. 

FIG.  77.— Photographs  showing  the  wedging  action  of  sand  in  forcing  apart 
the  coarse  aggregate  and  the  advantage  of  using  a  large  aggregate  which  reduces 
the  voids  and  the  surface  area  to  be  coated  with  cement  paste. 

Proportioning  by  Trial  Mixtures. — This  method  of 
proportioning  is  based  on  the  assumption  that  with  given 
materials  and  a  fixed  ratio  of  cement  to  total  aggregate 
that  mixture  of  cement,  water,  fine  aggregate  and  coarse 
aggregate  which  gives  the  least  volume  of  fresh  concrete 
will  give  the  densest  and  consequently  the  strongest  con- 
crete. Trial  mixtures  are  prepared  in  order  to  determine 
the  proportion  of  fine  aggregate  to  coarse  aggregate  which 
will  give  the  greatest  density  when  mixed  with  the  amount 
of  cement  to  be  used  on  the  work  and  with  the  amount  of 


214  MATERIALS  OF  ENGINEERING 

water  necessary  to  produce  a  concrete  which  is  workable 
and  easily  placed.  In  making  these  trial  mixtures  all  the 
materials  used  are  accurately  weighed  and  after  a  batch 
of  concrete  has  been  made  it  is  placed  a  little  at  a  time  in 
a  cylindrical  measure  of  constant  cross-section  and  carefully 
tamped  or  puddled.  After  all  the  material  is  in  the  cylin- 
der the  height  of  the  fresh  concrete  is  noted  and  recorded. 
The  concrete  is  then  removed  from  the  cylinder  and  the 
cylinder  is  cleaned.  A  new  batch  of  concrete  is  then 
prepared  using  the  same  weight  of  cement  and  water  as 
before  and  the  same  total  weight  of  fine  and  coarse  aggre- 
gate, but  changing  the  ratio  of  the  weight  of  the  fine  to  the 
coarse  aggregate.  This  new  batch  of  material  is  then 
placed  in  the  cylindrical  measure  and  the  height  of  the 
fresh  concrete  noted.  The  information  gained  after  a  few 
trials  will  serve  as  a  guide  in  making  up  other  trial  batches 
which  will  give  dense  mixtures.  The  process  is  repeated 
until  a  ratio  of  fine  to  coarse  aggregate  is  found  which 
gives  the  least  height  in  the  measure  and  at  the  same  time 
produces  a  workable  concrete  in  which  all  the  voids  are 
filled.  The  mixture  which  gives  the  least  height,  or  the 
smallest  yield,  and  at  the  same  time  gives  a  smooth- work- 
ing concrete  will  be  the  densest  which  can  be  easily  placed 
in  the  forms  and  should  give  relatively  high  strength. 
This  method  of  proportioning  when  carefully  carried  out 
will  give  accurate  results,  and  is  a  convenient  method  of 
determining  the  best  combination  of  natural  mixtures  of 
fine  and  coarse  aggregate. 

Mechanical  Analysis  and  its  Application  to  the  Propor- 
tioning of  Concrete. — In  the  various  methods  of  propor- 
tioning which  have  been  described  in  the  preceding  pages 
the  proportioning  is  done  by  what  maybe  termed  "rule 
of  thumb"  or  "cut  and  try"  methods.  In  these  methods 
of  proportioning  no  attempt  has  been  made  to  determine 
the  sizes  of  the  various  particles  composing  the  fine  and 
coarse  aggregate  for  the  purpose  of  studying  the  effect 
of  variations  in  the  sizes  of  particles  on  the  density,  strength 
and  other  properties  of  the  mortar  or  concrete.  Mechan- 


CONCRETE 


215 


ical  or  sieve  analysis  of  an  aggregate  consists  in  separating 
the  aggregate  into  the  various  sizes  of  particles  of  which  it 
is  composed.  It  gives  very  important  information  on  the 
grading  of  the  aggregate  and  it  enables  the  engineer  to 
make  intelligent  use  of  the  materials  at  his  disposal  when 
proportioning  them  for  concrete.  Mechanical  analysis 


Courtesy  of  the  W.  S.  Tyler  Co. 

FIG.  78. — Views  showing  a  series  of  testing  sieves  and  a  motor-driven  sieve 

shaker. 

is  also  of  great  aid  to  the  engineer  in  establishing  scientific 
methods  of  proportioning. 

In  making  a  mechanical  analysis  of  an  aggregate  a  repre- 
sentative sample  of  the  aggregate  is  taken  and  dried  after 
which  a  definite  weight  of  the  dry  aggregate  is  placed  on 


216  MATERIALS  OF  ENGINEERING 

the  uppermost  of  a  series  or  nest  of  sieves,  like  that  shown 
in  Fig.  78 (a).  Each  of  the  different  sieves  has  openings  of 
definite  size.  The  sieves  are  shaken  either  by  hand  or  by 
power  until  the  sample  of  aggregate  has  been  separated 
into  its  component  particles  by  the  sieves.  The  sieves 
used  are  so  arranged  that  the  coarsest  sieve,  the  one  having 
the  largest  openings,  is  at  the  top  of  the  series,  the  sieve 
of  next  largest  opening  being  placed  immediately  below  it 
and  so  on,  each  succeeding  sieve  having  smaller  openings 
than  the  sieve  immediately  above  it.  A  pan  is  provided 
at  the  bottom  of  the  series  to  catch  the  material  which 
passes  the  finest  sieve.  The  number  of  sieves  to  be  used 
in  making  mechanical  analyses  of  aggregate  depends 
largely  on  the  character  of  the  aggregate  to  be  tested  and 
the  use  to  be  made  of  the  results.  In  testing  fine  aggre- 
gates six  sieves  are  commonly  used.  In  testing  coarse 
aggregates  four  or  five  sieves  are  used.  A  sieve  which  is 
of  convenient  size  for  ordinary  use  is  8  in.  in  diameter  and 
2J4  m-  high.  Fig.  78(6)  shows  a  series  of  sieves  in  a  motor- 
driven  shaker  which  is  equipped  with  an  electric  time 
switch  for  stopping  the  shaker  automatically  after  a  pre- 
determined period  of  time. 

When  the  sample  of  aggregate  after  thorough  sieving 
has  been  separated  into  the  various  sizes  of  particles,  the 
amount  of  material  caught  or  retained  on  each  sieve  and  in 
the  pan  is  carefully  weighed.  Then  beginning  with  that 
caught  in  the  pan  the  weights  of  material  retained  on  the 
successive  sieves  are  added  and  each  sum  thus  obtained 
gives  the  total  weight  of  the  particles  which  have  passed 
through'  a  given  sieve.  For  convenience  in  interpreting 
and  plotting  the  results,  the  weights  of  material  passing 
a  given  sieve  are  expressed  as  percentages  of  the  total 
weight  of  material  used  in  the  analysis.  In  plotting  the 
results  of  an  analysis  the  sieve  openings  are  generally  plo  ted 
as  abscissas  and  the  percentages  passing  a  given  sievetare 
plotted  as  ordinates.  Fig.  79  illustrates  the  method  of 
recording  and  plotting  mechanical  analyses  of  fine  and 
coarse  aggregate. 


CONCRETE 


217 


The  curves  in  Fig.  79  are  called  mechanical  analysis 
curves.  They  show  graphically  the  sizes  of  the  particles 
composing  an  aggregate,  or  the  granulometric  composition 
of  the  aggregate  as  it  is  sometimes  called.  By  the  use  of 
such  curves  the  engineer  is  able  to  tell  what  sizes  of  par- 
ticles should  be  added  or  what  sizes  should  be  omitted  in 


Sieve 


Si5e 

^      oO       M- 


0.10        OJ5         0.20 
Diameters  of 


0          0.2 

Particles  in 


0.4         0.6 

Inches 


SIEVE    ANALYSES    OF  FINE  AGGREGATE 

SIEVE  ANALYSIS 

>  OF  COARSE  AGGREGATE 

MESH 

SIZE 
OF 

OPENING 
INCHES 

WEI6HT    OF    PARTICLES    IN   6RAMS 

PER  CENT  FINER  THAN 
EACH  SIEVE 

SIEVE 
SHE 

SIZE 
OF 

OPENIH6 

INCHES 

WEI6HTIN 

RETAINED 
OH 
SIEVES 

6RAMS 
PASSING 
EACH 
SIEVE 

PERCENT 
FINER 

THAN  EACH 
SIEVE 

RETAIN 

D  ON  SIEVES 

PASSING 

EACH  SIEVE 

FINE 
SAND 

MEDIUM 
SAND 

COARSE 
SAND 

FINE 
SAN'D 

MEDIUM 
SAND 

COARSE 
SAND 

FINE 
SAND 

MEDIUM 
SAND 

COARSE 
SAND 

3 

O263 

0 

0 

0 

1000 

1000 

1000 

IOO 

100 

100 

UN. 

I.05O 

0 

10000 

100 

4 

O.I  85 

0 

SO 

200 

1000 

950 

doo 

/OO 

95 

80 

3*« 

0.742 

1600 

8400 

04 

8 

0.  093 

40 

140 

300 

960 

8(0 

•500 

96 

Bl 

50 

>/z» 

0.525 

3400 

5000 

50 

14 

0.046 

70 

210 

330 

990 

600 

170 

89 

eo 

17 

£» 

0.371 

2500 

2500 

25 

20 

aot32 

160 

250 

130 

730 

350 

40 

73 

35 

4 

3  MESH 

0.263 

I&OO 

700 

7 

46 

0.0116 

440 

300 

290 

50 

20 

29 

5 

2 

4  MESH 

0.195 

400 

300 

3 

too 

0.0058 

240 

30 

50 

20 

10 

5 

2 

1 

&MESH 

0.093 

300 

0 

0 

PAH 

50 

20 

10 

PAN 

0 

FIG.  79. — Method  of  recording  and  plotting  sieve  analyses  of  fine  and    coarse 

aggregates. 

order  to  improve  the  grading  of  an  aggregate.  By  adding 
or  screening  out  certain  sizes  of  particles  he  is  able  to  ob- 
tain nearly  ideal  material  so  far  as  grading  of  the  material 
is  concerned.  Mechanical  ar  alyses  of  aggregates  thus  fur- 
nish the  engineer  with  information  concerning  the  materials 
which  is  of  great  aid  to  him  in  determining  what  proportions 
of  different  aggregates  will  give  the  best  results. 


218 


MATERIALS  OF  ENGINEERING 


Fuller  and  Thompson's  Method  of  Proportioning  Con- 
crete.— Mechanical  analysis  was  probably  first  made  use 
of  in  this  country  in  proportioning  concrete  materials  by 
W.  B.  Fuller  and  Sanford  E.  Thompson  who  nearly  twenty 
years  ago  made  extensive  tests  to  determine  what  combina- 
tions of  sizes  of  particles  of  fine  and  coarse  aggregates 
when  mixed  with  a  given  percentage  of  cement  by  weight 
would  give  the  densest  mixture.  In  the  experiments  gravel 
and  broken  stone  were  used  for  the  coarse  aggregate  and 
sand  and  screenings  were  used  for  the  fine  aggregate. 
Enough  water  was  added  to  the  various  mixtures  of  cement 


0.0 


0.1 


0.2 


0.&         Q9 


1.0 


Q3-        0.4         0.5         0.6         0.7 
Diameters   of  Parh'cles, Inches 

FIG.  80. — Sieve  analysis  curves  of  cement,  sand  and  gravel  also  ideal  and  com- 
bined curves  for  these  materials. 

and  graded  aggregate  to  produce  a  concrete  of  the  proper 
consistency.  After  mixing  the  materials  the  volume  of  the 
resulting  concrete  was  carefully  determined  and  from 
this  volume  the  density  of  the  concrete  was  calculated. 
It  was  found  that  for  the  materials  used,  the  mechanical 
analysis  curve  of  the  mixed  aggregates  and  cement  which 
produced  the  densest,  strongest  and  most  impermeable  con- 
crete resembled  a  parabola  and  consisted  of  a  combination 
of  an  ellipse  and  a  straight  line.  In  Fig.  80  is  shown  such 
an  ideal  curve  for  a  mixture  of  cement,  sand  and  gravel 
based  on  data  obtained  by  the  tests  of  Fuller  and  Thompson. 
The  combined  curve  shown  in  Fig.  80  was  drawn  using  the 
sieve  analyses  of  the  medium  sand  and  screened  gravel  given 


CONCRETE  219 

in  Fig.  79.  It  is  seen  that  by  mixing  the  cement,  sand  and 
gravel  in  the  proportions  1:2.2:4.8  by  weight,  the  ideal 
curve  is  fairly  closely  approached. 

For  the  proportions  used  in  the  tests  made  by  Fuller  and 
Thompson  which  were  1 : 9  by  weight,  the  average  gain  in 
strength  obtained  by  grading  the  aggregates  was  about 
14  per  cent.  The  statement  is  made  that  by  this  method 
of  proportioning  water-tight  concrete  has  been  secured 
with  graded  materials  mixed  in  the  proportions  of  one  part 
cement  to  three  parts  fine  aggregate  and  seven  parts  coarse 
aggregate,  whereas  for  water-tight  concrete  for  materials 
which  are  not  carefully  graded,  a  1:2:4  mix  is  generally 
used.  It  is  evident  that  by  careful  grading  a  considerable 
saving  in  cement  may  be  made.  In  general,  this  method 
of  proportioning  will  give  a  dense,  impermeable  concrete, 
and  on  large  construction  work  has  been  found  to  be  eco- 
nomical provided  the  cost  of  screening  and  handling  the 
available  aggregates  is  less  than  the  cost  of  the  additional 
cement  required  to  produce  concrete  of  equal  quality  from 
ungraded  material. 

Abrams'  Fineness  Modulus  Method  of  Proportioning 
Concrete. — In  Bulletin  1  of  the  Structural  Materials  Re- 
search Laboratory,  Lewis  Institute,  Chicago,  entitled  "De- 
sign of  Concrete  Mixtures/7  a  new  method  of  proportioning 
concrete  based  on  the  results  of  several  extensive  series 
of  tests  is  advanced  by  the  author,  Prof.  D.  A.  Abrams. 
The  basic  principle  of  Prof.  Abrams'  theory  of  proportion- 
ing is  that  with  given  concrete  materials  and  conditions 
of  test,  the  quantity  of  mixing  water  used  determines  the 
strength  of  the  concrete  so  long  as  the  mix  is  of  workable 
plasticity.  The  general  relation  between  the  compressive 
strength  and  the  water  content  of  concrete  as  determined 
by  Prof.  Abrams  from  the  results  of  an  elaborate  series  of 
tests  is  shown  in  Fig.  81.  The  results  given  in  Fig.  81 
cover  a  wide  range  of  mixes  and  for  each  mix,  except  the 
neat  cement,  the  maximum  size  of  the  aggregates  used 
ranged  from  that  which  passed  a  14-mesh  sieve  to  that 
which  passed  a  IK  in.  sieve.  Furthermore,  for  all  mixes 


220 


MATERIALS  OF  ENGINEERING 


and  gradings  there  was  a  wide  variation  in  the  amount  of 
water  used.  The  water  content  of  the  concretes  repre- 
sented in  Fig.  81  is  expressed  as  a  ratio  of  the  volume  of 
cement  used,  1  cubic  foot  of  cement  being  considered  to 
weigh  94  Ib.  The  legend  given  in  the  diagram  serves  to 
distinguish  the  various  mixes,  but  no  distinction  is  made 
either  between  the  aggregates  of  different  maximum  size 
or  between  the  different  consistencies  used.  The  data 
for  dry  concretes  or  concretes  which  were  not  easily  work- 
able are  not  included  in  the  diagram.  If  the  data  for  these 


6000 


Ratio  of  Volume  of  Yfa+er  to  Volume  of  Cement  -£-- 


Courtesy  of  D.  A.  Abrams. 

FIG.  81.  —  Relation  between  compressive  strength  of  concrete  and  water-cement 
ratio.     Twenty-eight  day  tests  of  6  X12-in.  cylinders. 

concretes  had  also  been  plotted  a  series  of  curves  (similar 
to  the  curve  shown  in  Fig.  93)  extending  downward  and 
to  the  left  from  the  main  curve  would  have  been  obtained. 
Prof.  Abrams  makes  use  of  sieve  analysis  in  proportion- 
ing the  aggregates  in  concrete  mixtures  and  has  found  that 
there  is  a  close  relation  between  the  size  and  grading  of  an 
aggregate  and  the  quantity  of  water  required  to  produce 
a  workable  concrete.  Prof.  Abrams  has  developed  a  method 
of  measuring  the  effective  size  or  grading  of  an  aggre- 
gate and  uses  for  this  purpose  a  function  which  he  teims 
fineness  modulus.  Fineness  modulus  is  computed  from 
the  sieve  analysis  of  the  aggregate  and  is  the  sum  of  the 


CONCRETE 


221 


percentages  in  the  sieve  analysis  of  the  aggregate  divided 
by  100.  In  making  the  sieve  analysis  the  following  sieves 
from  the  Tyler  standard  series1  are  used:  100,  48,  28,  14, 
8,  4,  %  in.,  Y±  in.,  and  1%  in.  Table  14,  which  is  taken 
from  Bulletin  1,  Structural  Materials  Research  Laboratory, 
Lewis  Institute,  gives  the  dimensions  of  the  sieves  used 
and  shows  the  method  of  computing  the  fineness  modulus 
of  fine  and  coarse  aggregates  and  also  of  a  mixture  of  fine 
and  coarse  aggregate.  It  should  be  noted  that  in  the  sieve 

TABLE  14. — METHOD  OF  CALCULATING  FINENESS  MODULUS  OF 
AGGREGATES  (ABRAMS) 

The  sieves  used  are  commonly  known  as  the  Tyler  standard  sieves. 
Each  sieve  has  a  clear  opening  just  double  that  of  the  preceding  one. 
The  sieve  analysis  may  be  expressed  in  terms  of  volume  or  \veight. 
The  fineness  modulus  of  an  aggregate  is  the  sum  of  the  percentages  given 
by  the  sieve  analysis,  divided  by  100. 


Size  of 

Sieve  analysis  of  aggregates 
Per  cent,  of  sample  coarser  than  a  given  sieve 

Sieve  size 

opening 

Sand 

Pebbles 

Concrete 

Fine 

(A) 

Medium 
(B) 

Coarse 

(C) 

Fine 
(D) 

Medium 
(E) 

Coarse 
(F) 

aggregate 
(G)» 

In. 

Mm. 

1  CO-mesh..  .0.0058 

0.147 

82 

91 

97 

100 

100 

100 

98 

48-mesh.  .  . 

0.0116 

0.295 

52 

70 

81 

100 

100 

100 

92 

28-mesh.  .  . 

0.0232 

0.59 

20 

46 

63 

100 

100 

100 

86 

14-mesh..  . 

0.046 

1.17 

0 

24 

44 

100 

100 

100 

81 

8-mesh.  .  . 

0.093 

2.36 

0 

10 

25 

100 

100 

100 

78 

4-mesh..  . 

0.185 

4.70 

0 

0 

0 

86 

95 

100 

71 

?s-in  

0.37 

9.4 

0 

0 

0 

51 

66 

86 

49 

?4-in  

0.75 

18.8 

0 

0 

0 

9 

25 

50 

19 

IM-in  1.5 

38.1 

0 

0 

0 

0 

0 

0 

0 

Fineness  modulus  

1.54         2.41           3.10 

6.46 

6.86 

7.36 

5.74 

2  Concrete  aggregate  "G"  is  made  up  of  25  per  cent,  sand  "B"  mixed  with  75  per  cent, 
of  pebbles  "E."  Equivalent  gradings  would  be  secured  by  mixing  33  per  cent,  sand  "B" 
with  67  per  cent,  coarse  pebbles  "F";  28  per  cent.  "A"  with  72  per  cent.  "F,"  etc.  The 
proportion  coarser  than  a  given  sieve  is  made  up  by  the  addition  of  these  percentages  of  the 
corresponding  size  of  the  constituent  materials. 

analyses  given  in  Table   14  the  per  cent,  of  the  sample 
coarser  than  a  given  sieve  is  recorded  instead  of  the  per 

1  A  series  of  sieves  made  of  square-mesh,  wire  cloth  by  the  W.  S.  Tyler 
Company  of  Cleveland,  Ohio. 


222 


MATERIALS  OF  ENGINEERING 


cent,  finer  than  a  given  sieve  as  was  done  in  Fig.  79.  The 
fineness  modulus  of  an  aggregate  may  also  be  calculated 
by  dividing  the  sum  of  the  percentages  passing  the  sieves 
(provided  the  sieves  given  in  Table  14  are  used)  by  100 
and  subtracting  the  result  from  the  number  of  sieves  in  the 
standard  set.  For  example,  the  fineness  modulus  of  the 
medium  sand  given  in  Fig.  79  is,  omitting  the  No.  3  sieve, 

100  +  100  +  100  +  95  +  81  +  60  +  35  +  5  +  2\ 


9.0- 


100 


3.22. 


5000 


4.00 


7.00 


4.50  5.00  5.50  6.00  6.50 

Fineness   Modulus  of  Aqqreqate 

Courtesy  of  D.  A.  Abrams. 
FIG.  82. — Relation  between  fineness  modulus   of  aggregate   and    compressive 

strength  of  concrete. 

Sand  and  pebble  aggregate  graded  0-1  >£  in.,  28-day  tests  of  6  by  12-in.  cylinders.  Nor- 
mal consistency  (R  =  1.00). 

Prof.  Abrams  states  that  mixtures  of  given  fine  and 
coarse  aggregate  which  have  the  same  fineness  modulus 
when  mixed  with  a  given  quantity  of  cement  require  the 
same  quantity  of  water  to  produce  concrete  of  the  same 
workability  and  give  concrete  of  the  same  strength  so  long 
as  they  are  not  too  coarse  for  the  quantity  of  cement  used. 
Fineness  modulus  thus  furnishes  a  means  of  judging  of  the 
concrete  making  qualities  of  an  aggregate. 

The  relation  between  the  fineness  modulus  of  the  mixed 
aggregate  and  the  compressive  strength  of  the  resulting 


CONCRETE 


223 


concrete  for  four  concrete  mixes  is  shown  in  Fig.  82.  The 
graphs  given  in  Fig.  82  show  that  as  the  fineness  modulus 
increases  the  strength  increases  until  the  higher  values 
of  fineness  modulus  are  reached  when  the  strength  begins 
to  decrease.  Fig.  82  also  shows  that  the  point  of  maximum 
strength  occurs  at  higher  values  of  fineness  modulus  for  the 
rich  mixes  than  for  the  lean  mixes. 

Fig.  83  shows  the  relation  between  fineness  modulus  and 
strength  for  a  mixture  of  1  part  cement  to  5  parts  aggre- 
gate, by  volume,  when  the  maximum  size  of  the  aggregate 


JT 

i.    3000 

o-/V 

.     >/bc 

•P  " 

&_ 

.0 

_j 

.E 

+-  eooo 

1-5  Mix, 

by  Volum 

* 

j^X 

cp  f- 

1 

m 
>    1000 

0-4 

^/ 

x<V 

iL 

o          o 

00-  14-  Ac 

igregate 

1. 

00            Z 

00         3 

00          4-. 

DO          5. 

DO           6. 

DO          7.( 

)0          8.C 

Fineness  Modulus  of 

Courtesy  of  D.  A.  Abrams. 
FIG.  83. — Relation  between   fineness   modulus   of  aggregate   and    compressive 

^strength  of  concrete. 
28-day  tests  of6  by  12  in-  cylinders.     Normal  consistency  (R  =  1.00).! 

is  varied  without  changing  the  type  of  the  sieve-analysis 
curve.  It  will  be  noted  in  Fig.  83  that  the  strength  in- 
creases as  the  maximum  size  of  the  aggregate  is  increased 
and  that  the  height  to  which  the  curve  rises  appears  to  be 
limited  only  by  the  maximum  size  of  aggregate  which 
may  be  used.  For  the  range  in  sizes  and  for  the  mixes 
ordinarily  used  the  strength  of  the  concrete  may  be  assumed 
to  increase  directly  with  fineness  modulus. 

Since  the  quantity  of  mixing  water  used  greatly  influences 
the  strength  of  the  resulting  concrete  it  is  important 
that  the  water  be  carefully  proportioned.  The  quantity 
of  water  to  be  used  may  be  calculated  by  the  following 
formula  which  Prof.  Abrams  has  derived  from  the  results 


224  MATERIALS  OF  ENGINEERING 

of  numerous  tests  and  which  takes  into  account  all  of  the 
factors  which  affect  the  quantity  of  water  required  in  a 
concrete  mixture: 

.30n  1    , 
P  +  T-O^--    -rV(a-e)n 


where   x  =  ratio  of  the  volume  of  water  to  the  volume  of  cement  in 
the  mix  (water-cement  ratio)  . 

R  =  relative  consistency  or  "  workability  factor."  Normal 
consistency  (relative  consistency  =  1.00)  requires  the 
use  of  such  a  quantity  of  mixing  water  as  will  cause  a 
shortening  of  %  to  1  in.  in  a  freshly  molded  6  X  12  in. 
cylinder  of  concrete  upon  removing  by  a  steady  upward 
pull  the  smooth  metal  mold  in  which  it  is  cast.  A  relative 
consistency  of  1.10  means  that  approximately  10  per  cent. 
more  water  than  that  required  for  normal  consistency  has 
been  used. 

p  =  amount  of  water  required  by  the  cement  to  produce  a  paste 
of  normal  consistency  in  the  standard  test  for  cement, 
expressed  as  a  ratio  of  the  weight  of  the  cement. 

m  =  fineness  modulus  of  the  mixed  aggregate  (used  as  an  ex- 
ponent in  the  formula). 

n  =  ratio  of  the  volume  of  the  mixed  aggregate  to  the  volume 
of  the  cement  in  the  mix. 

•  a  =  ratio  of  the  volume  of  the  water  absorbed  by  the  dry 
aggregate  to  the  volume  of  the  aggregate,  after  immersion  in 
water  for  three  hours.  (An  average  value  for  broken 
limestone  and  for  gravel  may  be  taken  as  0.02.  Porous 
sandstones  may  absorb  0.08  and  a  very  light  and  porous 
aggregate  may  absorb  0.25). 

c  =  ratio  of  the  volume  of  the  moisture  contained  in  the  mixed 
aggregate  to  the  volume  of  the  aggregate.  (For  room 
dry  aggregate  c  =  0.) 

A  simpler  form  of  the  above  equation  which  is  sufficiently 
accurate  for  ordinary  ranges  of  mix  and  grading  of  aggre- 
gate is  as  follows: 


(O32n-™j)]+(a 


c)n 


Design  of  Concrete  Mixtures  by  Abrams'  Fineness 
Modulus  Method.  —  In  designing  concretes  by  this  method 
the  aim  is  to  find  that  combination  of  the  given  materials 
which  for  a  given  water-cement  ratio  will  produce  a  work- 


CONCRETE  225 

able  mixture  with  the  use  of  a  minimum  of  cement.  In 
the  bulletin  previously  referred  to  Prof.  Abrams  has  out- 
lined the  steps  to  be  followed  in  designing  concrete  mixes 
which  are  substantially  as  follows: 

"1.  Knowing  the  approximate  compressive  strength  required  of  the 
concrete,  estimate  the  driest  workable  "relative  consistency"  which 
may  be  used.  Experience  or  trial  is  the  only  guide  in  determining 
the  relative  consistency  of  concrete  necessary  in  the  work.  A  rela- 
tive consistency  of  1.00  (normal  consistency)  is  somewhat  dry  for 
most  concrete  work,  but  can  be  used  where  light  tamping  is  prac- 
ticable. A  relative  consistency  of  1.10  represents  the  driest  concrete 
which  can  be  satisfactorily  used  in  concrete  road  construction.  A 
relative  consistency  of  1.20  will  generally  be  found  satisfactory  for 
reinforced  concrete  construction.  A  relative  consistency  of  1.25  rep- 
resents about  the  wettest  consistency  which  should  be  used  in  rein- 
forced concrete  work.  The  size  of  the  aggregate  available,  or  which 
must  be  used,  and  other  factors  will  furnish  a  guide  as  to  the  mix. 
The  mix  is  expressed  as  one  volume  of  cement  to  a  given  number  of 
volumes  of  aggregate;  that  is,  of  the  combined  fine  and  coarse  aggregate. 

"2.  Make  sieve  analysis  of  fine  and  coarse  aggregate,  using  Tyler 
standard  sieves  of  the  following  sizes:  100,  48,  28,  14,  8,  4,  %,  %,  \Y2 
in.  Express  the  sieve  analysis  in  terms  of  the  percentages  of  material 
by  weight  (or  separate  volumes)  coarser  than  each  of  the  standard  sieves. 

"3.  Compute  fineness  modulus  of  each  aggregate  by  adding  the 
percentages  found  in  (2). 

"4.  Determine  the  maximum  size  of  aggregate  by  applying  the 
following  rules:  If  more  than  20  per  cent,  of  aggregate  is  coarser  than  any 
sieve  the  maximum  size  shall  be  taken  as  the  next  larger  sieve  in  the 
standard  set;  if  between  11  and  20  per  cent,  is  coarser  than  any  sieve, 
the  maximum  size  shall  be  the  next  larger  "half  sieve";  if  less  than  10 
per  cent,  is  coarser  than  certain  sieves,  the  smallest  of  these  sieve  sizes 
shall  be  considered  the  maximum  size. 

"5.  Assume  a  mix  and  from  Table  15  determine  the  maximum  value 
of  fineness  modulus  which  may  be  used  for  the  mix,  kind  and  size  of 
aggregate,  and  the  work  under  consideration. 

"6.  Compute  the  percentages  of  fine  and  coarse  aggregates  required 
to  produce  the  fineness  modulus  desired  for  the  final  aggregate  mixture 
by  applying  the  formula: 


in  which  y  =  percentage  of  fine  aggregate  in  total  mixture; 
A  =  fineness  modulus  of  coarse  aggregate; 
B  =  fineness  modulus  of  final  aggregate  mixture; 
C  =  fineness  modulus  of  fine  aggregate. 
15 


226  MATERIALS  OF  ENGINEERING 

TABLE  15 

MAXIMUM   PERMISSIBLE  VALUES   OF  FINENESS   MODULUS   OF 
AGGREGATES1  (ABRAMS) 

For  mixes  other  than  those  given  in  the  table,  use  the 
values  for  the  next  leaner  mix. 

For  maximum  sizes  of  aggregate  other  than  those  given 
in  the  table,  use  the  values  for  the  next  smaller  size. 

Fine  aggregate  includes  all  material  finer  than  No.  4 
sieve;  coarse  aggregate  includes  all  material  coarser  than 
the  No.  4  sieve.  Mortar  is  a  mixture  of  cement,  water  and 
fine  aggregate. 

This  table  is  based  on  the  requirements  for  sand-and- 
pebble  or  gravel  aggregate  composed  of  approximately 
spherical  particles,  in  ordinary  uses  of  concrete  in  reinfor- 
ced concrete  structures.  For  other  materials  and  in  other 
classes  of  work  the  maximum  permissible  values  of  fineness 
modulus  for  an  aggregate  of  a  given  size  is  subject  to  the 
following  corrections: 

(1)  If  crushed  stone  or  slag  is  used  as  coarse  aggregate, 
reduce  values  in  table  by  0.25.     For  crushed  material  con- 
sisting of  unusually  flat  or  elongated  particles,  reduce  values 
by  0.40. 

(2)  For  pebbles  consisting  of  flat  particles,  reduce  values 
by  0.25. 

(3)  If  stone  screenings  are  used  as  fine  aggregate,  reduce 
the  values  by  0.25. 

(4)  For  the  top  "course  in  concrete  roads  reduce  values 
by  0.25.     If  finishing  is  done  by  mechanical  means,  this 
reduction  need  not  be  made. 

(5)  In  work  of  massive  proportions,  such  that  the  smallest 
dimension  is  larger  than  10  times  the  maximum  size  of  the 
coarse  aggregate,  additions  may  be  made  to  the  values  in 
the  table  as  follows:  for  %-in.  aggregate  0.10;  for  IM-in. 
0.20;  for  3-in.  0.30;  for  6-in.  0.40. 

1  It  has  been  found  by  test  that  there  is  a  maximum  practicable  value  of 
fineness  modulus  for  each  size  of  aggregate  and  mix.  It  is  necessary  there- 
fore to  place  certain  limits  on  the  value  of  fineness  modulus  which  may 
be  used  for  proportioning  materials  for  concrete  mixes.  Table  15  gives 
the  limits  which  will  be  found  practicable,  and  the  purpose  of  the  table 
is  to  avoid  the  attempt  to  secure  an  aggregate  grading  which  is  too  coarse 
for  its  maximum  size  and  for  the  amount  of  cement  used. 


CONCRETE 


227 


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228  MATERIALS  OF  ENGINEERING 

11 7.  With  the  estimated  mix,  fineness  modulus  and  consistency 
enter  Fig.  841  and  determine  the  strength  of  concrete  produced  by 
the  combination.  If  the  strength  shown  by  the  diagram  is  not  that 
required,  the  necessary  readjustment  may  be  made  by  changing  the 
mix,  consistency  or  size  and  grading  of  the  aggregates." 

To  illustrate  the  use  of  the  chart  given  in  Fig.  84  the  following 
example  is  given:  It  is  desired  to  know  the  approximate  strength  of 
a  1 : 3  mortar  of  1.20  relative  consistency  made  with  a  sand  having  a  fine- 
ness modulus  3.00.  Draw  a  line  through  mix  1 : 3  and  fineness  modulus 
3.00,  as  indicated  by  the  upper  dotted  line  in  the  chart  and  mark  the  point 
where  it  intersects  the  "  reference  line  for  consistency."  Through  this 
point  of  intersection  draw  a  horizontal  line  and  read  the  strength  at 
the  point  where  this  horizontal  line  crosses  the  vertical  line  for  relative 
consistency  1.20.  In  this  case  the  strength  is  approximately  2,200 
Ib.  per  sq.  in.  Repeating  this  process  using  the  lower  dotted  line  in 
the  chart  it  will  be  found  that  a  1 : 5  mix  made  with  an  aggregate  having 
a  fineness  modulus  of  5.70  has  a  strength  of  approximately  2,400 
Ib.  per  sq.  in.  at  relative  consistency  1.20  and  is  therefore,  slightly 
stronger  than  the  1 : 3  mortar  given  in  the  previous  example.  At  normal 
consistency  the  strength  of  the  1:5  mix  would  be  approximately 
3,300  Ib.  per  sq.  in.,  or  nearly  1,000  Ib.  per  sq.  in.  stronger  than  the 
same  mix  at  relative  consistency  1.20.  It  is  evident  that  in  this  case 
a  marked  increase  in  strength  is  obtained  by  the  use  of  the  smaller 
quantity  of  mixing  water. 

The  development  of  the  fineness  modulus  method  of  pro- 
portioning adds  much  to  our  knowledge  of  the  proper 
use  of  materials  when  proportioning  them  for  concrete. 
This  method  of  proportioning  is  significant  in  that  it  is 
the  first  method  to  call  attention  to  the  marked  influence 


1  IMPORTANT  NOTE. — It  must  be  understood  that  the  values  in  Fig.  84 
were  determined  from  compression  tests  of  6  by  12-in.  cylinders  stored  for 
28  days  in  a  damp  place.  The  values  obtained  on  the  work  will  depend 
on  such  factors  as  the  consistency  of  the  concrete,  quality  of  the  cement, 
methods  of  mixing,  handling,  placing  the  concrete,  etc.,  and  on  age  and 
curing  conditions. 

"Strength  values  higher  than  those  given  for  relative  consistency  of  1.10 
should  seldom  be  considered  in  designing,  since  it  is  only  in  exceptional 
cases  that  a  consistency  drier  than  this  can  be  satisfactorily  placed.  For 
wetter  concrete  much  lower  strengths  must  be  considered."  In  general, 
some  allowance  must  be  made  for  the  high  strengths  obtained  in  labora- 
tory tests. 


CONCRETE 


229 


of  mixing  water  on   the  strength  of  concrete  made  with 
given    materials.     The  fineness  modulus  method  of  pro- 


Reference  Line 
for  Co  mis  fancy  •-' 


030  1.00    1.10    1.20  1.30  140    1.50  IjfcO  1.70 
Relative  Consistency 

Courtesy  of  D.  A.  Abrams. 

FIG.  84. — Diagram  for  the  design  of  concrete  mixtures. 
This  chart  is  based  on  compression  tests  of  6  by  12-inch  concrete  cylinders; 
age  28  days;  stored  in  damp  sand. 

The  cement  used  gave  compressive  strengths  in  1-3  standard  sand  mortar  as 
follows,  when  tested  in  the  form  of  2  by  4  inch  cylinders. 


Age 
7  days 

Lb.  per  sq.  in. 
1  900 

28  days 

3  200 

3  months. 

4,200 

1  vear  .  . 

4.300 

portioning    affords    a   convenient   means   of   judging  the 
mortar-making  and  the  concrete-making  qualities  of  aggre- 


230  MATERIALS  OF  ENGINEERING 

gates,  is  of  wide  application,  and  when  used  intelligently 
should  effect  considerable  economy  of  material  and 
labor. 

Edwards'  Surface  Area  Method  of  Proportioning  Mortar 
and  Concrete. — Mr.  L.  N.  Edwards  in  a  paper  presented 
in  1918  before  the  American  Society  for  Testing  Materials 
proposed  a  new  method  of  proportioning  the  materials 
of  mortars  and  concretes  which  he  termed  the  ''Surface 
Area"  method  of  proportioning.  The  basic  principle  of 
this  method  of  proportioning  assumes  that  the  strength 
and  other  properties  of  mortars  and  concretes  are  mainly 
dependent  upon  the  amount  of  cementing  material  used 
in  relation  to  the  total  surface  area  of  the  aggregates.  Mr. 
Edwards  found  from  tests  which  he  made  that  with  ma- 
terials of  uniform  quality  this  method  of  proportioning 
provided  a  means  of  securing  uniformly  strong  mortars  and 
concretes  from  sands  of  varying  granulometric  composition 
and  that  the  strength  of  mortars  was  dependent  upon  (1) 
the  quantity  of  cement  used  in  relation  to  the  surface  area 
of  the  aggregates  and  (2)  the  consistency  of  the  mix.  Mr. 
Edwards  also  found  that  the  compressive  and  tensile 
strengths  of  mortars  of  a  uniform  normal  consistency  made 
with  sands  of  varying  granulometric  composition  was  pro- 
portional to  the  quantity  of  cement  used  in  relation  to  the 
surface  area  of  the  aggregate  and  that  the  amount  of  water 
required  to  produce  mortars  of  normal  consistency  was 
a  function  of  the  weight  of  cement  used  and  the  total  sur- 
face area  of  the  sand  to  be  wetted. 

Mr.  Edwards  determined  the  approximate  average  sur- 
face areas  of  sand  and  stone  particles  of  varying  sizes  in 
the  following  manner :  First  a  sieve  analysis  of  the  material 
was  made  and  the  average  number  of  particles  in  given 
weights  of  the  material  retained  on  each  sieve  were  counted. 
Knowing  the  number  of  particles  in  a  given  weight  and 
the  specific  gravity  of  the  particles,  the  average  volume 
of  each  size  of  particle  was  computed.  The  surface  areas 
of  the  particles  were  than  determined  from  the  average 


CONCRETE 


231 


volume  of  the  various  sizes  of  particles  and  the  shape1 
of  the  particles.  From  the  data  thus  obtained  the  surface 
area  of  a  given  weight  of  particles  of  a  given  size  was  easily 
calculated. 

In  making  practical  application  of  this  method  of  pro- 
portioning, the  work  of  determining  the  surface  area  of  the 


PerCent ,or  Weighf  of  Sand  (\  b) 


0         10       20       30       40       50 

Per  Cent,  or  Weight  of  Gravel  (!  b.) 


Courtesy  of  R.  B.  Young. 
FIG.  85. — Relation  between  grading  of  pit-run  sand  and  gravel  and  surface  area 

aggregate  and  the  quantity  of  cement  to  be  used  in  a  given 
case  is  much  simplified  by  the  use  of  tables,  or  diagrams 
like  those  shown  in  Fig.  85  and  Fig.  86,  which  are  prepared 
from  sieve  analyses  and  tests  of  the  materials. 

In  an  article  in  Engineering  News-Record  by  R.  B. 
Young,  a  description  is  given  of  the  use  of  the  surface- 
area  method  in  proportioning  concrete  on  actual  construc- 


1  In  the  calculation  of  surface  area,  sand  and  gravel  particles  were  assumed 
as  spherical.  The  particles  of  broken  stone  were  assumed  to  be  made  up 
of  one-third  cubical  and  two-thirds  parallelopipedal  shapes  the  latter  being 
sub-divided  in  order  to  approximate  more  closely  the  areas  of  the  flat, 
elongated  shapes  of  the  broken  stone  particles. 


232 


MATERIALS  OF  ENGINEERING 


tion.  The  diagrams  in  Fig.  85,  which  were  taken  from 
Mr.  Young's  article,  show  the  relation  between  the  grading 
of  pit-run  sand  and  gravel  and  surface  area.  The  diagrams 
in  Fig.  86  show  the  relation  which  Mr.  Young  found 
between,  the  compressive  strength  and  cement  content, 
and  the  compressive  strength  and  water-cement  ratio 
for  concrete  of  normal  consistency  using  given  materials. 


* 

L 

0.4000 

1 

P     »«An 

j 

\ 

/ 

\ 

A 

\ 

;ompre&sive  Strength,  F 
—  ro  c 

§  i 

y 

^ 

J 

\ 

s 

1 

' 

\ 

/ 

<«) 

^ 

y^ 

("> 

A 

"--*. 

— 

Cement  Content  Pounds  Water-Cement  Ratio 

per  100  5q.  Ft.  of  Surface 
Area 

Courtesy  of  R.  B.  Young. 

FIG.  86. — Curves  showing  the  relation  between  compressive  strength  and 
cement  content  of  concrete  at  normal  consistency;  and  between  compressive 
strength  and  water-cement  ratio. 


In  using  this  method  of  proportioning  in  the  field  the 
surface  area  of  an  economical  combination  (previously 
determined  by  laboratory  tests)  of  the  given  fine  and  coarse 
aggregates  is  obtained  from  the  diagrams  in  Fig.  85  after  sieve 
analyses  of  the  aggregates  in  their  natural  condition  have  been 
made.  Knowing  the  minimum  compressive  strength  desired 
for  the  class  of  construction  in  which  the  concrete  is  to  be 
used,  the  amount  of  cement  required  to  give  this  strength  is 
determined  from  the  diagram  shown  in  Fig.  86 (a).  Gener- 
ally a  margin  of  safety  of  about  300  Ib.  per  sq.  in.  is  allowed 
to  take  care  of  field  conditions.  When  the  amount  of 
cement  to  be  used  has  been  determined  the  amount  of 
mixing  water  is  calculated  from  the  data  given  in  Fig. 
86(6)  and  from  a  consideration  of  the  minimum  water- 


CONCRETE  233 

cement  ratio  which  can  be  used  for  the  class  of  concrete 
desired.  Allowance  is  made  for  moisture  contained  in  the 
aggregates  in  calculating  the  amount  of  water  to  be  used. 
If  the  consistency  of  the  concrete  is  found  to  be  too  dry 
for  the  work  under  consideration  the  water  content  and  the 
cement  content  of  the  mix  are  increased  in  the  same  pro- 
portion until  the  required  mobility  is  obtained.  In  case 
the  consistency  is  wetter  than  necessary  the  cement  and 
water  are  reduced  in  the  same  manner.  By  increasing  or 
decreasing  the  cement  and  water  in  the  same  proportion 
the  proper  water-cement  ratio  necessary  to  insure  ob- 
taining concrete  of  the  desired  strength  is  maintained. 
In  order  to  have  a  check  on  the  strength  of  the  concrete 
test  cylinders  of  standard  size  are  made  from  concrete 
taken  from  the  forms  during  the  pouring  operation.  The 
cylinders  are  tested  after  curing  for  28  days. 

Concrete  proportioned  by  the  surface-area  method  has 
been  found  by  tests  to  be  uniform  in  quality  and  to  give  the 
required  strength  and  it  is  claimed  that  the  use  of  this 
method  of  proportioning  has  effected  a  considerable  saving 
in  cement. 

Comparison  of  Methods  of  Proportioning  Concrete. 
Proportioning  by  arbitrary  selection  of  volumes,  by  voids 
in  aggregate,  or  by  trial  mixtures  can  be  carried  out  in  the 
field  without  making  a  laboratory  study  of  the  available 
aggregates.  As  has  been  stated,  these  methods  of  pro- 
portioning are  not  scientific  and  may  often  result  in  a  need- 
less waste  of  cement.  When  used  by  engineers  having 
experience  in  the  proportioning  of  concrete  these  methods 
will  usually  give  satisfactory  results.  Proportioning  by  the 
Fuller  and  Thompson  method  involves  sieve  analysis 
of  the  aggregates  to  be  used.  This  method  of  proportion- 
ing gives  reasonable  insurance  of  a  dense,  strong  concrete 
when  the  aggregates  used  are  similar  in  character  to  those 
used  in  the  tests  upon  which  the  method  is  based.  Some- 
times this  method  requires  expensive,  artificial  grading 
of  aggregate.  The  fineness  modulus  method  and  the 
surface  area  method  of  proportioning  also  necessitate 


234  MATERIALS  OF  ENGINEERING 

sieve  analysis  and  some  involved  computations,  but  are  of 
wide  application  and  appear  to  insure  a  uniform  concrete 
of  good  quality  without  excessive  cost  of  artificial  grading 
of  aggregate.  The  methods  of  proportioning  which  in- 
volve sieve  analysis  of  aggregate  may  be  expected  to  effect 
economy  of  materials  when  used  on  work  of  considerable 
magnitude.  The  limitations  of  the  various  methods  should 
be  carefully  observed,  and  the  fact  always  borne  in  mind 
that  the  freshly  mixed  concrete  must  be  of  such  a  consist- 
ency that  it  can  be  easily  and  effectively  placed  in 
the  forms. 

Mixing  Concrete. — When  the  proportions  of  the  con- 
crete materials  have  been  determined  by  one  of  the  fore- 
going methods  of  proportioning,  the  proper  quantities  of 
cement,  water,  and  aggregates  for  the  various  batches 
of  concrete  should  be  carefully  measured  in  order  to  insure 
a  uniform  product.  Cement  is  usually  measured  by  weight, 
a  bag  of  cement  which  weighs  94  Ib.  being  considered  the 
equivalent  of  one  cubic  foot.  A  barrel  of  cement  contains 
four  bags  and  is  considered  the  equivalent  of  four  cubic 
feet.  The  fine  and  coarse  aggregate  or  the  mixed  aggregate 
is  measured  by  loose  volume.  On  small  jobs  the  aggre- 
gates are  measured  either  in  a  bottomless  box  of  known 
volume  placed  on  a  wheel  barrow,  or  in  wheel  barrows 
which  hold  definite  volumes  of  the  material  when  filled  to 
a  given  height  in  a  given  manner.  On  large  structures 
the  aggregates  are  usually  measured  in  hoppers  of  known 
capacity. 

The  quality  and  amount  of  water  used  in  mixing  concrete 
is  of  great  importance.  The  water  should  be  free  from  oil, 
acid,  alkali  and  organic  matter.  Good  results  will  be 
obtained  when  the  amount  of  water  used  will  give  the 
concrete  such  a  consistency  that  it  will  flow  sluggishly 
into  the  forms.  Concrete  made  too  wet  is  low  in  density 
and  strength  and  when  an  excess  of  water  is  used  there  is 
danger  of  separation  of  the  coarse  aggregate  from  the 
mortar  paste  while  the  concrete  is  being  conveyed  to  the 
forms.  Concrete  made  too  dry  will  not  fill  all  corners  of  the 


CONCRETE  235 

forms  and  will  not  flow  around  the  reinforcing  bars  as 
readily  as  will  concrete  of  the  proper  consistency.  For  any 
given  mixture  the  proper  proportion  of  water  should  be 
determined  by  calculation  and  trial  and  the  water  used 
should  be  carefully  measured.  The  water  for  the  con- 
crete is  generally  measured  in  buckets  when  hand-mixing 
is  used.  Machine  mixers  are  provided  with  small  tanks 
having  a  device  for  indicating  the  amount  of  water  dis- 
charged into  the  mixer. 

Mixing  the  ingredients  for  concrete  may  be  done  either 
by  hand  or  in  a  power-driven  mixer.  Hand-mixing,  which 
is  used  only  on  small  jobs,  is  done  on  a  water-tight  plat- 
form. In  hand-mixing,  the  fine  aggregate  is  spread  out  in  a 
long  flat  pile  on  the  mixing  platform  and  covered  with  the 
cement.  The  sand  and  cement  are  then  turned  over  at 
least  three  times  with  shovels.  The  coarse  aggregate  is 
then  added  and  the  mass  is  hollowed  to  form  a  long  crater 
into  which  the  mixing  water  is  poured.  After  adding  the 
proper  quantity  of  water  the  ingredients  are  again  turned 
over  and  over  with  shovels  until  the  mass  is  uniform  in  color 
and  appearance.  The  mass  should  be  turned  over  not  less 
than  six  times  after  the  water  is  added. 

Concrete  mixers  are  of  two  types,  continuous  mixers  and 
batch  mixers.  In  continuous  mixers  the  aggregate,  the 
cement,  and  the  water  are  fed  into  one  end  of  a  trough 
and  forced  along  the  trough  to  the  other  end  by  means  of 
rotating  screw-paddles  which  mix  the  ingredients  as  they 
move  along  the  trough.  The  proportioning  of  cement  and 
aggregate  is  determined  by  the  rate  of  supply  of  each. 
Continuous  mixers  can  be  operated  cheaply  and  rapidly, 
but  rarely  give  a  uniform  product. 

In  batch  mixers  definite  amounts  of  aggregate,  cement, 
and  water  are  placed  inside  a  revolving  drum,  which  is 
fitted  with  blades  projecting  inward.  As  the  drum  revolves 
the  ingredients  are  constantly  agitated  by  the  blades  and 
are  carried  part  way  round  and  dropped  back  to  the  bot- 
tom of  the  drum.  A  thorough  mixing  is  thus  effected. 
A  batch  mixer  should  be  run  for  at  least  1H  to  2  minutes 


236 


MATERIALS  OF  ENGINEERING 


before  the  concrete  is  discharged,  the  exact  time  of  mixing 
depending  on  the  size  of  the  mixer  used.  In  Fig.  87  are 
plotted  some  results  of  tests  made  by  Prof.  Abrams  at 
Lewis  Institute  to  determine  the  effect  of  time  of  mixing  on 
the  compressive  strength  of  various  concrete  mixes.  The 
curves  in  Fig.  87  show  that  for  the  mixer  used  there  was 
a  fairly  rapid  increase  in  strength  with  increase  in  time  of 
mixing  until  the  2-minute  period  was  reached.  Mixing  be- 
yond] the  2-minute  period  generally  gave  but  little  increase 


5000 


10 


3456789 
Time  of  Mixing  in   Minutes 

Courtesy  of  D.  A.  Abrams. 

FIG.  87. — Relation  between  compressive  strength  of  concrete  and  time  of  mixing. 
All  aggregates  graded  0-1  Y±  inches.     1.10  relative  consistency. 

in  strength.  Batch  mixers  properly  handled  produce  very 
uniform  concrete  mixtures.  Fig.  88  shows  the  general  ap- 
pearance of  a  batch  mixer. 

Handling  and  Placing  Concrete. — After  the  concrete 
has  been  mixed  it  should  be  quickly  transported  in  such  a 
manner  as  will  prevent  separation  of  the  ingredients  to 
the  place  where  it  is  to  be  deposited.  On  small  structures 
wheel-barrows  or  two-wheeled  carts  are  generally  used  to 
transport  the  concrete  to  the  forms.  On  large  structures 
cars,  cableways  and  towers  with  inclined,  open  chutes  are 


CONCRETE 


237 


used.  When  towers  and  chutes  are  used  the  concrete  is 
hoisted  to  a  hopper  placed  in  the  tower  at  a  considerable 
height  above  the  work.  The  concrete  is  then  fed  from  the 
hopper  into  the  chutes  at  a  uniform  rate  and  it  flows  by 
gravity  either  directly  to  the  place  of  deposit  or  to  another 
hopper  from  \vhich  it  is  wheeled  into  place  with  barrows  or 
small  carts.  Frequently  in  chuting  concrete  the  tendency 
is  to  use  an  excess  of  mixing  water  in  order  to  cause  the 
concrete  to  flow  down  the  chute  readily.  The  use  of  excess 


FIG.  88. — Batch  mixer  for  concrete. 

water  often  causes  the  coarser  particles  of  aggregate  to 
separate  from  the  mortar.  Due  to  this  separation  of  the 
ingredients  and  to  the  fact  that  an  excess  of  mixing  water 
has  a  very  harmful  influence  on  the  strength  of  concrete, 
this  method  of  handling  often  produces  a  concrete  which  is 
not  uniform  and  which  is  of  decidedly  poor  quality. 

In  depositing  concrete  under  water  special  methods  have 
to  be  used  in  order  to  prevent  the  cement  from  being  washed 
away  and  also  to  prevent  separation  of  the  various  ingre- 


238  MATERIALS  OF  ENGINEERING 

dients.  A  tremie  is  generally  used  to  deposit  concrete 
under  water.  A  tremie  consists  of  a  tube  one  foot  or  more 
in  diameter  provided  with  a  hopper  at  the  top  and  a  slightly 
flaring  bottom,  which  is  filled  with  concrete  and  then  low- 
ered into  place  with  a  derrick.  The  tube  is  kept  full  of  con- 
crete at  all  times  and  to  cause  a  flow  of  concrete  through  the 
tube  the  discharge  end  is  raised  a  few  inches  at  a  time  and 
moved  slowly  about  in  the  newly  placed  concrete.  Con- 
crete to  be  deposited  under  water  is  sometimes  lowered 
into  place  in  a  drop-bottom  bucket  or  in  loosely-woven 
bags  of  jute  or  other  coarse  cloth.  To  prevent  washing 
of  the  cement  from  the  mixture  and  to  prevent  spreading 
of  the  concrete,  cofferdams  are  frequently  used  to  enclose 
the  space  in  which  the  concrete  is  to  be  deposited. 

Before  concrete  is  deposited  in  the  forms  all  debris  should 
be  removed  from  the  place  to  be  occupied  by  the  concrete. 
The  forms  should  be  cleaned  and  then  thoroughly  wetted 
(except  hi  freezing  weather)  to  prevent  absorption  of 
water  from  the  concrete.  Frequently  the  surface  of  the 
forms  exposed  to  the  concrete  are  oiled  in  order  to  prevent 
warping  and  to  aid  in  removing  the  forms  from  the  con- 
crete after  it  has  hardened. 

To  secure  dense  and  uniform  concrete  it  should  be  placed 
in  the  forms  evenly  and  without  separation  of  the  ingre- 
dients. Concrete  which  is  of  a  fairly  dry  consistency  should 
be  placed  in  layers  about  6  or  8  inches  in  thickness.  For 
wetter  consistencies  the  thickness  of  the  layers  may  be 
increased  to  12  inches  or  more  depending  on  the  shape  and 
size  of  the  section  being  poured.  After  placing,  the  con- 
crete is  puddled  and  spaded  to  remove  air  bubbles  and  to 
aid  in  securing  a  dense,  uniform  mixture.  To  give  a  smooth 
surface  and  to  prevent  visible  voids  in  the  finished  concrete 
a  spading  tool  is  run  up  and  down  between  the  form  and  the 
fresh  concrete.  This  permits  the  escape  of  air  and  also 
works  the  mortar  to  the  forms  thus  giving  a  smooth  finish 
to  the  exposed  surfaces  of  the  mass.  The  same  result 
may  also  be  obtained  by  pounding  the  forms  with  wooden 
mauls  or  by  vibrating  the  forms  with  pneumatic  hammers. 


CONCRETE  239 

In  making  concrete  pavements  power-driven  tampers 
and  light,  metal  rollers  are  often  used  to  compact  the  con- 
crete and  to  aid  in  securing  a  dense  product. 

When  concrete  is  to  be  deposited  on  or  against  concrete 
which  has  set,  the  surface  of  the  set  concrete  is  roughened 
and  cleaned  of  all  foreign  matter  and  laitance  (a  light- 
colored,  powdery  substance  which  forms  on  the  surface  of 
concrete  which  has  been  mixed  too  wet)  after  which  it  is 
wetted  with  water.  The  wetted  surface  is  then  thoroughly 
coated  with  a  wash  of  neat  cement  grout,  or  a  rich  mortar, 
before  the  new  concrete  is  applied. 

Curing  of  Concrete. — A  considerable  amount  of  water  is 
required  for  the  proper  hydration  of  the  cement,  which  goes 
on  for  several  days  after  the  concrete  has  become  hard. 
It  should  be  borne  in  mind  that  thin  sheets  of  concrete 
if  exposed  to  the  air,  dry  out  so  rapidly  that  some  of  the 
water  necessary  for  the  complete  hydration  of  the  cement 
may  be  evaporated.  In  order,  therefore,  that  concrete 
may  set  and  harden  properly  after  it  has  been  placed  it  is 
very  essential  that  it  be  protected  in  some  way  to  prevent 
it  from  drying  out  too  rapidly.  The  result  of  too  rapid 
drying  out  of  concrete  is  the  failure  of  the  concrete  to  gain 
strength  normally  with  the  lapse  of  time,  moreover  shrink- 
age stresses  are  set  up  which  may  cause  serious  cracks  in 
the  suiface  of  the  concrete.  Sprinkling  two  or  three  times 
a  day,  covering  with  canvas  which  is  wetted  from  time  to 
time  and  covering  with  moist  sand  or  earth  are  methods 
employed  to  insure  proper  curing  conditions.  In  warm 
weather  the  concrete  is  kept  wet  for  at  least  two  weeks  after 
pouring.  Allowing  the  forms  to  remain  in  place  helps 
to  retain  the  moisture  in  the  concrete  and  prevents  too 
rapid  drying.  Newly  placed  concrete  pavements  are 
sometimes  protected  from  the  direct  rays  of  the  sun  by 
means  of  light,  wooden  frames  covered  with  canvas.  After 
the  concrete  has  thoroughly  set  the  pavement  is  covered 
with  moist  earth  or  with  water.  To  hold  the  water  on  the 
pavement  small  dikes  of  earth  are  built  along  the  sides  of  the 
pavement  and  also  across  the  pavement  at  frequent  intervals. 


240 


MATERIALS  OF  ENGINEERING 


In  Fig.  89  are  plotted  the  results  of  tests  made  at  the 
University  of  Illinois  on  concrete  cylinders  ,of  different 
consistencies  which  were  stored  under  different  conditions. 
The  graphs  in  Fig.  89  show  that  the  air-stored  specimens 
of  normal  consistency  gained  but  little  in  strength  with 
lapse  of  time.  On  the  other  hand,  for  the  specimens  of 
normal  consistency  stored  in  damp  sand,  there  was  a  grad- 
ual increase  in  strength  with  age.  This  was  also  true  of 
the  specimens  of  other  consistencies  which  were  stored  in 
damp  sand.  At  the  age  of  five  years  the  air-stored  speci- 


Series  D  -  Dry  Consistency  8.4%  Wahr.,  Stored  in  Damp  Sand  A & 

Series  S- Normal  Consistency  9.3%Water.,  Stored  in  Damp  Sand  -o        o- 
Series  P- Normal  Consistency  9.3%Water.,Paraffined-StoredinDamp$ando—Q 

Series  W-WetConsistency-JO.2%  Water.,  Stored  in  Damp  Sand  --• •-. 

Series  A  ~  Normal  Consistency  9.3%  Water,  Stored  in  Air  —  + ^ 


FIG.  89. 


-Effect  of  storage  condition  on  compressive  strength  of  concrete  at 
various  ages.     Proportions,  1:2>2:3}^,  by  weight. 


mens  were  only  about  one-half  as  strong  as  the  specimens 
of  like  consistency  stored  in  damp  sand.  Attention  is 
called  to  the  considerable  increase  in  strength  with  age 
of  the  specimens  which  were  transferred  from  air-storage 
to  damp-sand  storage  when  they  were  two  years  three 
months  old. 

A  good,  general  rule  for  concrete  construction  is  to  use 
as  little  water  for  mixing  as  will  give  a  workable  concrete, 
and  to  use  plenty  of  water  on  the  concrete  after  it  has  set. 

Effect  of  Low  Temperature  on  Newly  Made  Concrete.— 
Concrete  subjected  to  low  temperatures  while  being  poured 


CONCRETE 


241 


or  within  a  few  days  after  being  poured  is  greatly  reduced 
in  strength.  The  hydration  of  the  cement,  with  conse- 
quent hardening  of  the  concrete,  is  retarded,  and  if  actual 
freezing  of  the  concrete  takes  place  the  strength  may  be 
permanently  impaired.  Fig.  90  based  on  the  results  of 
tests  by  A.  B.  McDaniel  at  the  University  of  Illinois  shows 
the  general  effect  of  low  temperature  on  the  strength  of 
concrete.  When  it  is  necessary  to  lay  concrete  in  cold 
weather  the  bad  effects  of  low  temperature  of  the  surround- 


024       68       10       12      14      16       18 

Aqe  -  Days 

FIG.  90. — Effect  of  temperature  on  the  strength  of  concrete.    From  Bulletin  81 , 
Eng.  Exp.  Station,  Unir.  of  Illinois  by  A.  B.  McDaniel. 

ing  air  may  be  minimized  by  heating  the  stone,  the  sand, 
and  the  water  used,  and  by  covering  the  concrete  as  soon 
as  it  is  laid  with  canvas,  burlap,  straw,  sawdust,  or  manure. 
Structures  poured  in  cold  weather  are  often  enclosed  in 
canvas,  the  enclosed  space  being  heated  for  a  few  days  with 
small  open  stoves  or  with  pipe  lines  supplied  with  live  steam, 
in  order  that  the  concrete  may  set  and  harden  properly.1 

1  The  addition  of  salt  to  the  mixing  water  lowers  the  freezing  temperature 
of  concrete  but  increases  the  danger  of  disintegration,  especially  if  there  is 
danger  of  electrolysis  by  stray  electric  currents.  In  general  the  use  of  salt 
or  other  chemicals  to  prevent  freezing  is  not  to  be  recommended. 

16 


242  MATERIALS  OF  ENGINEERING 

Molds  and  Forms  for  Concrete. — The  units  which 
make  up  a  concrete  structure,  beams,  columns  and  the  like, 
are  sometimes  cast  separately,  and  afterward  fitted  together 
to  form  the  structure.  This  is  the  unit  system  of  casting. 
In  the  unit  system  of  casting  concrete  it  is  frequently 
feasible  to  use  metal  forms  and  by  their  use  to  produce 
members  of  fine  surface  finish  which  are  very  uniform 
in  size.  In  structures  made  up  of  separately  cast  units 
the  strength  of  the  joints  between  members  must  be  care- 
fully considered. 

Concrete  blocks  for  building  walls,  concrete  drain  tile, 
and  concrete  sewer  pipe  are  made  in  metal  molds.  Such 
concrete  units  are  frequently  made  of  very  dry  concrete, 
(See  Fig.  93  page  247)  which,  by  thorough  tamping  or 
pressing,  is  given  sufficient  rigidity  so  that  it  can  be  re- 
moved from  the  mold  in  a  few  minutes  after  tamping. 
The  molded  block  or  pipe  unit  is  transferred  to  a  room 
where  it  is  exposed  to  steam  or  to  water-saturated  air  for 
a  few  days,  after  which  it  has  gained  sufficient  strength 
to  be  transferred  to  a  storage  yard  for  further  curing. 
Concrete  blocks  are  usually  made  hollow  and  are  of  a  great 
variety  of  shapes  and  sizes. 

When  a  concrete  structure  is  made,  not  of  separate 
units,  but  of  one  mass  of  concrete  it  is  said  to  be  mono- 
lithic (literally  " single-stone")-  In  monolithic  concrete 
construction  forms,  usually  of  wood,  are  set  up  for  a  con- 
siderable section  of  the  complete  structure,  and  the  con- 
crete is  mixed  and  poured  into  these  forms.  After  a  period 
of  10  days  or  2  weeks  (a  longer  period  may  be  necessary  in 
cold  weather)  the  forms  are  removed  leaving  the  concrete 
structure  in  place.  In  constructing  concrete  buildings  the 
forms  are  set  up  for  a  story  at  a  time.  Fig.  91  shows  a 
building  being  constructed  one  story  at  a  time  by  the  mono- 
lithic system.  For  the  lower  seven  stories  the  concrete 
has  hardened  sufficiently  to  allow  the  removal  of  the  forms, 
in  the  eighth  story  are  seen  the  timbers  supporting  the 
floor  of  the  ninth  story,  in  the  ninth  story  the  concrete 
t>een  poured,  but  has  not  hardened  sufficiently  to  allow 


CONCRETE 


243 


the  removal  of  forms;  on  the  roof,  concrete  is  being  poured. 
The  removal  of  forms  from  a  monolithic  concrete  structure 
should  be  done  very  carefully.  Many  serious  accidents 
have  been  caused  by  the  premature  removal  of  forms. 


Courtesy  of  Leonard  Construction  Co. 

FIG.  91. — Reinforced  concrete  building  under  construction. 

Before  removal  of  the  forms  an  examination  of  the  surface 
of  portions  of  the  concrete  should  be  made  to  make  sure 
that  it  has  attained  a  good  degree  of  hardness,  then  sup- 
ports and  forms  should  be  removed  over  a  small  area  only, 


244 


MATERIALS  OF  ENGINEERING 


while  careful  watch  should  be  kept  for  signs  of  undue 
settlement  or  deflection  of  the  concrete  as  its  support  is 
removed.  If  no  signs  of  failure  are  observed  more  forms 
may  be  removed. 

Strength  of  Concrete. — Concrete  is  a  brittle  material 
and  like  all  brittle  materials  is  stronger  in  compression 
than  in  tension  with  a  strength  in  shear  intermediate 

5000 


4000 


4=   3000 


2000 


1000 


Specimens  made  in  Laboratory  ana 

stored  in  Damp  Sand 
Aqqreaate    River  Sand  and  Soft 

Limestone 

Based  on  Tests  made  at  the  University 
of  Illinois 


567 
Age  -  Mon+hs 

FIG.  92. — Strength  of  concrete  of  varying  age  and  proportions  of  cement  to 

aggregate. 

The  proportions  given  for  the  aggregate  are  the  volumes  of  the  sums  of  the 
fine  and  coarse  aggregates,  which  were  measured  separately. 

The  modulus  of  elasticity  of  concrete  varies  from  about  1,000,000  Ib.  per  sq.  in. 
to  4,000,000  Ib.  per  sq.  in.,  averaging  about  2,000,000  Ib.  per  sq.  in.  for  concrete 
made  up  of  1  part  cement  to  6  parts  aggregate. 

between  the  tensile  strength  and  the  compressive  strength. 
The  tensile  strength  of  concrete  is  so  low  (100-300  Ib. 
per  sq.  in.)  that  it  is  not  considered  in  the  design  of  re- 
inforced-concrete  structures.  The  shearing  strength  of 
concrete  is  of  importance  because  shearing  stress  is  set  up 


CONCRETE 


245 


in  the  concrete  near  the  supports  of  reinforced-concrete 
beams,  in  footings,  and  around  columns  in  flat  slab  floors. 
The  strength  of  the  bond  which  exists  in  reinforced  con- 
crete between  the  concrete  and  the  steel  is  of  great  im- 
portance since  lack  of  bond  will  prevent  the  two  materials 
from  working  together  as  intended  and  thus  seriously 
impair  the  strength  of  structural  members. 

For  concrete  made  of  good  materials  under  normal 
temperature  conditions  three  very  important  factors  in 
determining  its  compressive  strength  are:  (1)  age,  (2) 
richness  of  the  concrete  in  cement  and  (3)  amount  of  mixing 
water  used.  Well  made  concrete  tends  to  grow  stronger 
with  the  passage  of  time,  but  the  rate  of  increase  of  strength 
diminishes  rapidly  after  a  few  weeks.  Fig.  92  shows  the 
change  of  strength  with  time  for  several  mixtures  of  con- 
crete. The  test  specimens  were  made  under  laboratory 
conditions  and  stored  in  damp  sand.  From  the  diagrams 
it  may  be  seen  that  concrete  1  year  old  is  about  2.5  times 
as  strong  as  concrete  1  month  old  and  is  about  twice  as 
strong  as  concrete  2  months  old.  Under  ordinary  con- 
ditions concrete  can  not  be  counted  on  to  gain  much 
strength  after  a  year.  Fig.  92  also  shows  the  general  effect 
of  the  proportion  of  cement  upon  the  strength  of  concrete. 
Table  16  is  from  the  report  of  the  Joint  Committee  on 

TABLE  16. — STRENGTH  OF  PORTLAND  CEMENT  CONCRETE  IN  COMPRESSION 

The  values  given  are  from  the  report  of  the  Joint  Committee  on  Concrete 
and  Reinforced  Concrete.  They  are  based  on  data  of  test  specimens  in  the 
form  of  cylinders  8  in.  in  diameter  by  16  in.  long,  made  and  stored  under 
laboratory  conditions  and  tested  when  28  days  old.  All  values  are  in  pounds 
per  square  inch. 


Aggregate 

Proportion  of  cement  to  aggregate 

l:3i 

l:4.5i 

l:6i 

l:7.5i 

l:Qi 

Granite,  trap  rock  

3,300 

3,000 
2,200 
800 

2,800 
2,500 
1,800 
700 

2,200 
2,000 
1,500 
600 

1,800 
1,600 
1,200 
500 

1,400 
1,300 
1,000 
400 

Gravel,  hard  limestone,  hard  sandstone  .  . 
Soft  limestone  soft  sandstone          

Cinders                                                       .  • 

1  Total  volume  fine  and  coarse  aggregates  measured  separately. 


246  MATERIALS  OF  ENGINEERING 

Concrete  and  Reinforced  Concrete  and  gives  values 
which  should  be  obtained  for  concrete  1  month  old  when 
made  with  good  workmanship  and  good  materials.  For 
concrete  which  is  to  be  subjected  to  direct  compression 
the  simplest  and  cheapest  way  to  add  strength  is  in  nearly 
all  cases  to  increase  the  proportion  of  cement  in  the  mixture. 
Table  17  gives  the  average  results  of  a  large  number  of  tests 
made  at  the  University  of  Illinois  on  the  strength  of  con- 
crete in  shear.  The  average  strength  in  shear  is  slightly 
greater  than  one-half  of  the  compressive  strength  for 
rich  concrete  and  a  somewhat  greater  proportion  of  the 
compressr^e  strength  of  lean  concrete. 

TABLE  17. — STRENGTH  OF  PORTLAND  CEMENT  CONCRETE  IN  SHEAR 

The  values  given  below  summarize  test  results  given  in  Bulletin  No.  8  of 
the  Engineering  Experiment  Station  of  the  University  of  Illinois.  All  con- 
crete was  made  and  stored  under  laboratory  conditions  and  tested  when  60 
days  old.  The  aggregate  used  was  torpedo  bank  sand  and  soft  limestone. 


Proportion  of  cement  to 
aggregate 

Ultimate  Ib.  per  sq.  in. 

Ratio  of  strength  in  shear 
to  strength  in  compression 

Shear 

Compression 

1.-61 

1,290 

2,430 

0.532 

liQ1 

1,090 

1,290 

0.842 

1  Total  volume  fine  and  coarse  aggregates  measured  separately. 

The  curve  shown  in  Fig.  93,  which  is  plotted  from  the 
results  of  tests  made  by  Prof.  Abrams  at  Lewis  Institute, 
shows  the  general  relation  between  the  strength  of  concrete 
and  the  amount  of  mixing  water  used.  The  curve  in  Fig. 
93  is  a  composite  curve  which  summarizes  the  results  of 
compression  tests  on  various  mixtures  of  cement  and  mixed 
aggregate,  the  grading  of  the  mixed  aggregate  being  the 
same  for  all  mixes.  It  will  be  noted  that  the  strength 
increases  rapidly  with  increase  in  the  quantity  of  mixing 
water  over  the  range  on  the  curve  indicated  by  AB.  With 
further  increase  in  the  amount  of  water  the  strength  falls 
off  rapidly  as  indicated  by  the  portion  of  the  curve  BCDEF. 
Concretes  made  with  the  quantity  of  water  represented  by 
the  [portion  of  the  curve  AB  are  too  dry  and  stiff  for  most 


CONCRETE 


247 


purposes  but  could  be  used  in  making  building  blocks, 
drain   tile  and  other  concrete  products  requiring  a   dry 


90 

f>» 

£  . 

f 

\ 

\ 

\ 

\ 

Per  Cent  of  MaximumStr 
o  o  8  8  &  8  S  c 

i 

\ 

\ 

i 
i 

\ 

\° 

\ 

v 

^«*» 

"^x. 

e 

^ 

•^ 

^^, 

F 

70     &0    90      100     110      120     130      140     150      160     170     180     190    ZOO 
Water  U&ed-  Per  Cent  of  Quantity  Giving  Maximum  Strength 

Courtesy  of  D.  A.  Abrams. 
FIG.  93. — Effect  of  quantity   of  mixing  water  on  strength  of  concrete. 


7000 


Ae 


Mix.   1-4 

at  Te&t  4  Mont 


o  Hbbles 
•  Limesrone 


0.4     0.6       0.&       1.0       1.2       1.4 

Wear  in  Inches 


1.6      2.0       22. 


Courtesy  of  D.  A.  Abrams. 
FIG.  94. — Relation  between  compressive  strength  and  wear  of  concrete. 

mixture.     The  proper  amount  of  water  for  concrete  road 
work  corresponds  to  that  at  C.     The  amount  of  water 


248  MATERIALS  OF  ENGINEERING 

used  in  building  construction  very  frequently  corresponds 
to  that  portion  of  the  curve  from  D  to  F  and  it  is  evident 
that  only  about  30  per  cent,  of  the  available  strength  is  ob- 
tained with  such  wet  mixtures.  In  general,  in  reinforced 
concrete  construction  good  results  will  be  obtained  when 
the  amount  of  water  used  produces  a  concrete  which  will 
flow  sluggishly  into  the  forms. 

Fig.  94  which  is  also  plotted  from  the  results  of  tests  made 
by  Professor  Abrams  shows  the  relation  between  the  com- 
pressive  strength  and  the  wear  for  a  concrete  mixture 
such  as  is  commonly  used  in  making  concrete  roads  and 
pavements.  It  is  seen  that  the  amount  of  wear  decreases 
rapidly  with  increase  in  the  strength  of  the  concrete. 
The  amount  of  wear  of  the  concrete  in  these  tests  was 
determined  by  testing  blocks  of  concrete  in  a  Talbot-Jones 
rattler  with  an  abrasive  charge  of  cast-iron  balls. 

Results  of  tests  of  strength  of  the  bond  between  concrete 
and  steel  bars  are  given  in  Fig.  95  which  is  plotted  from 
tests  made  by  D.  A.  Abrams  at  the  University  of  Illinois. 
The  values  given  are  the  loads  per  square  inch  of  embedded 
surface  of  bar  which  cause  marked  slip  between  plain  round 
steel  bars  and  various  mixtures  of  concrete.  There  are 
on  the  market  various  forms  of  special  reinforcing  bars  for 
reinforced  concrete  which  are  rolled  with  projections  which 
are  for  the  purpose  of  giving  the  bars  a  positive  anchorage 
in  the  concrete.  The  tests  of  Abrams  show  that  the  effect- 
iveness of  such  anchorage  lies  in  its  holding  power  after 
some  slip  has  taken  place  rather  than  in  any  tendency  to 
prevent  slip  altogether.  The  following  quotation  from 
Abrams'  published  results  gives  the  requirements  for  a 
well-designed,  special  reinforcing  bar,  or  " deformed  bar" 
as  it  is  sometimes  called. 

"In  a  deformed  bar  of  good  design  the  projections  should  present 
bearing  faces  as  nearly  as  possible  at  right  angles  to  the  axis  of  the 
bar.  The  areas  of  the  projections  should  be  such  as  to  preserve  the 
proper  ratio  between  the  bearing  stress  against  the  concrete  ahead 
of  the  projections  and  the  shearing  stress  over  the  surrounding  envelope 
of  concrete.  Failure  by  shearing  of  the  concrete  should  be  avoided. 


CONCRETE 


249 


The  tests  indicate  that  the  areas  of  the  projections  measured  at  right 
angles  to  the  axis  of  the  bar  should  not  be  less  than  say,  20  per  cent,  of 
the  superficial  area  of  the  bar.  A  closer  spacing  of  the  projections 
than  is  used  in  commercial  deformed  bars  would  be  of  advantage. 
Advocates  of  the  deformed  bar  would  do  well  to  recognize  the  fact  that 
in  a  deformed  bar  which  may  be  expected  to  develop  a  high  bond 
resistance,  a  certain  amount  of  metal  must  be  used  in  the  projections 
which  probably  will  not  be  available  for  taking  tensile  stress." 
800 


700 


74  Plain  Rounds 
Embedment   8" 


Bond  Stress  at  End  Slip  of  0.0005 

Maximum.  Bond  Resistance 


20         40          60         80  100         120         140         160         180        200 


FIG.  95. — Strength  of  bond  between  concrete  and  steel.       (From  Bulletin    71, 
Enj.  Exp.  Station,  Univ.  of  Illinois  by  D.  A.  Abrams. 

Working  Stresses  in  Concrete. — The  following  statement 
of  allowable  working  stresses  gives  the  values  recom- 
mended in  the  report  of  the  Joint  Committee  on  Concrete 
and  Reinforced  Concrete. 

Allowable  axial  unit  stress  in  compression  on  concrete 
piers  and  short  columns,  22.5  per  cent,  of  the  ultimate  com- 
pressive  strength  of  the  concrete. 


250  MATERIALS  OF  ENGINEERING 

Allowable  unit  stress  in  the  extreme  compression  fibers  of 
beams,  32.5  per  cent,  of  the  ultimate  compressive  strength 
of  the  concrete  (adjacent  to  the  supports  of  continuous  beams 
this  value  may  be  increased  to  37.5  per  cent.). 

Allowable  unit  stress- in  shear  (punching),  6  per  cent, 
of  the  ultimate  compressive  strength  of  the  concrete. 

Allowable  unit  stress  in  bond  between  concrete  and  steel 
reinforcing  bars;  for  plain  bars,  4  per  cent,  of  the  ultimate 
compressive  strength  of  the  concrete;  for  drawn  wire,  2 
per  cent,  of  the  ultimate  compressive  strength  of  the 
concrete;  for  the  best  types  of  "deformed"  bars,  5  per  cent, 
of  the  ultimate  compressive  strength  of  the  concrete. 

Disintegration  of  Concrete,  Waterproofing. — A  few  struc- 
tures of  concrete  and  of  reinforced  concrete  have  dis- 
integrated under  the  action  of  sea  water,  of  alkali  water,  of 
frost,  or  under  the  action  of  electrolysis  from  stray  electric 
currents  from  street  railway  and  electric  lighting  systems. 
In  such  cases  the  disintegrating  action  seems  to  be  due 
mainly  to  the  porosity  of  the  concrete  and  to  the  presence 
of  free  moisture  in  the  concrete.  Frequently  poor  work- 
manship and  the  use  of  poor  materials  have  also  contributed 
to  the  failure  of  concrete  structures  by  disintegration. 
Structures  in  which  a  properly  placed,  dense  concrete 
made  of  good  materials  was  used  have  been  little  affected 
by  the  above  agencies.  The  simplest  way  to  insure  a 
concrete  which  will  resist  disintegration  is  to  use  a  rela- 
tively high  proportion  of  cement  and  carefully  selected, 
well-graded  aggregates. 

A  dense  concrete  resists  the  percolation-  of  water  through 
it.  A  1:2:4  concrete  made  with  well-graded  aggregate 
is  practically  watertight  for  ordinary  pressures.  In  gen- 
eral, the  impermeability  or  watertightness  of  concrete  in- 
creases with  increase  in  amount  of  cement,  with  increase 
in  maximum  size  of  aggregate,  with  increase  in  thickness 
of  wall,  with  decrease  of  pressure,  and  with  increase  in  age. 
Other  important  factors  which  affect  the  watertightness 
of  concrete  are:  the  consistency  of  the  mix,  the  thorough- 
ness of  mixing,  the  manner  of  placing  and  compacting 


CONCRETE  251 

the  concrete  and  the  manner  of  curing  the  concrete.  It 
is  essential  that  cracking  of  the  concrete  due  to  shrinkage 
in  setting  and  to  temperature  changes  be  avoided  in  water- 
tight construction.  Consequently  reinforcing  steel  and 
expansion  joints  must  be  generally  be  provided  to  aid  in 
preventing  the  formation  and  extension  of  cracks. 

Various  waterproofing  compounds  to  be  mixed  with  con- 
crete materials,  and  waterproofing  coatings  to  be  spread  on 
concrete  are  on  the  market.  Some  of  these  are  effective 
when  carefully  used  but  in  many  cases  the  use  of  a  rich, 
dense  concrete  of  the  proper  consistency  will  render  the 
structure  practically  waterproof,  and  resistant  to  disinte- 
grating influences. 

Use  of  Concrete  for  Fireproofing. — Concrete  makes 
excellent  fireproofing  material  for  steel  columns  and  girders 
and  structures  made  of  concrete  are  very  resistant  to 
destruction  by  fire.  Under  the  action  of  heat  the  surface 
of  the  concrete  is  dehydrated,  and  the  evaporation  of  the 
water  chemically  combined  with  the  cement  keeps  down 
the  temperature  of  the  inner  layers  of  concrete.  The  de- 
hydrated surface  of  the  concrete  is  rendered  weak  and 
porous  by  the  heat  but  the  injury  rarely  extends  over  an 
inch  or  two  into  the  concrete;  moreover,  the  dehydrated 
surface  is  an  excellent  heat  insulator,  and  affords  increased 
protection  to  the  inner  layers  of  concrete.  From  2  to  2}^ 
inches  of  good  concrete  is  generally  considered  sufficient 
thickness  of  fireproofing  for  steel  work.  In  reinforced 
concrete  structures,  columns  and  girders  are  usually  pro- 
tected by  a  minimum  thickness  of  2  in.  of  concrete,  beams 
and  walls  by  a  minimum  of  l^J  in.  and  floor  slabs  by  a 
minimum  of  1  in.  A  study  of  the  action  of  reinforced 
concrete  buildings  when  subjected  to  fire,  and  fire  tests  of 
reinforced  concrete  columns  under  load  indicate  that  ag- 
gregates made  up  of  highly  silicious  materials,  such  as 
sandstones,  pebbles,  quartz  and  granite  pebbles,  are 
less  resistant  to  the  action  of  heat  than  are  aggregates 
such  as  limestone,  trap  rock,  blast-furnace  slag  and 
burned  clay.  Experience  and  tests  also  show  that  round 


252  MATERIALS  OF  ENGINEERING 

columns  are  less  seriously  affected  by  fire  than  are  square 
columns. 

One  marked  advantage  which  concrete  possesses  as  a 
fireproofing  material  for  steel  lies  in  the  fact  that  its  co- 
efficient of  expansion  is  very  nearly  the  same  as  that  of 
steel,  so  that  there  is  less  danger  of  sp ailing  off  under  the 
action  of  heat  for  concrete  fireproofing  than  there  is  for 
fireproofing  materials  whose  coefficients  of  expansion 
differ  widely  from  that  of  steel. 

Selected  References  for  Further  Study 

TAYLOR  AND  THOMPSON:  "Concrete,  Plain  and  Reinforced,"  New  York, 
1916.  A  standard  treatise  by  two  American  engineers. 

TURNEAURE  AND  MAURER:  "Principles  of  Reinforced  Concrete  Construc- 
tion," New  York,  1919.  A  mathematical  treatment  of  the  mechanics 
of  reinforced  concrete. 

JOHNSON:  "Materials  of  Construction,"  Fifth  Edition  Rewritten  by  M.  O. 
Withey  and  James  Aston,  New  York,  1918.  Chapters  XIII,  .XFV, 
and  XV. 

HOOL:  "Principles  of  Reinforced  Concrete  Construction"  (three  volumes), 
New  York,  1916. 

HOOL  AND  JOHNSON:  "Concrete  Engineers'  Handbook,"  New  York,r1918. 

TALBOT:  Bulletins  on  Concrete  and  Reinforced  Concrete,  University  of 
Illinois  Engineering  Experiment  Station,  Bulletins  1,  4,  8,  10,  12,  14, 
20,  22,  28,  29,  56,  64,  67,  84,  106. 

U.  S.  BUREAU  OP  STANDARDS:  Technologic  Papers  2,  3,  5,  12,  58,  95. 

MCDANIEL:  Influence  of  Temperature  on  the  Strength  of  Concrete,  Uni- 
versity of  Illinois  Engineering  Experiment  Station,  Bulletin  81. 

ABRAMS:  Tests  of  Bond  between  Concrete  and  Steel.  University  of  Illinois 
Engineering  Experiment  Station  Bulletin  71.  Also  Bulletins  1,  2,  3,  4, 
Structural  Materials  Research  Laboratory,  Lewis  Institute,  Chicago. 

ABRAMS  AND  HARDER:  "  Colorimetric  Test  for  Organic  Impurities  in  Sands," 
Circular  1,  Structural  Materials  Research  Laboratory,  Lewis  Institute, 
Chicago. 

ABRAMS:  "Effect  of  Time  of  Mixing  on  the  Strength  and  Wear  of  Concrete," 
Proceedings  of  the  American  Concrete  Institute  (1918). 

ABRAMS:  "Effect  of  Age  on  the  Strength  of  Concrete,"  Proceedings  of  the 
American  Society  for  Testing  Materials,  Vol.  XVIII,  Part  II,  (1918). 

HULL:  "Fire  Tests  of  Concrete  Columns."  Proceedings  of  the  American 
Concrete  Institute.  Vol.  XIV  (1918)  and  Vol.  XV  (1919). 

FULLER  AND  THOMPSON:  "The  Laws  of  Proportioning  Concrete,"  Transac- 
tions, American  Society  of  Civil  Engineers,  Vol.  59  (1907),  page  67. 

EDWARDS:  "Proportioning  the  Materials  of  Mortars  and  Concretes  by  Sur- 
face Areas  of  Aggregate,"  Proceedings  American  Society  for  Testing 
Materials,  Vol.  XVIII,  Part  II  (1918). 


CONCRETE  253 

YOUNG:  "Mixing  Concrete  by  Surface  Areas  on  Actual  Work,"  Engineering 
News-Record,  Vol.  84,  No.  1,  page  33,  Jan.  1,  1920. 
"Analysis  of  Concrete- Proportioning  Theories,"   Canadian  Engineer, 
Vol.  37,  No.  22,  page  487,  Nov.  27,  1919. 

CRUM:  "Proportioning  of  Pit-Run  Gravel  for  Concrete,"  Proceedings 
American  Society  for  Testing  Materials,  Vol.  XIX,  Part  II  (1919). 

Report  of  Joint  Committee  on  Concrete  and  Reinforced  Concrete,  Trans- 
actions of  the  American  Society  of  Civil  Engineers,  Vol.  LXXXI,  page 
1101,  (1917).  Proceedings  of  the  American  Society  for  Testing  Ma- 
terials, Vol.  XVII,  Part  I  (1917;. 


CHAPTER  XIX 
RUBBER,  LEATHER,  BELTING,  ROPE 

Rubber,  General  Characteristics. — The  special  field  of 
usefulness  of  rubber  as  an  engineering  material  depends  on 
three  salient  characteristics:  (1)  its  value  as  an  electrical 
insulator,  (2)  its  impermeability  to  water  and  gases,  and 
(3)  its  ability  to  withstand  great  deformation  without 
serious  structural  damage.  As  an  electrical  insulator 
rubber  is  used  in  very  large  quantities  for  insulating  cover- 
ing for  electric  wires  and  for  insulating  bushings  and  plates. 
The  waterproof  and  gas  proof  qualities  of  rubber  make  it 
widely  used  for  hose  for  water,  for  compressed  air  and  for 
gas,  and,  together  with  its  ability  to  withstand  great  de- 
formation, make  it  the  material  universally  used  for 
pneumatic  tires  for  vehicles.  Its  ability  to  withstand 
great  deformation  makes  it  useful  in  members  whose  func- 
tion it  is  to  "take  up"  shocks  in  machines  and  structures, 
such  as  buffers  and  tires  for  vehicle  wheels. 

Production  of  Rubber. — All  the  rubber  in  commercial 
use  is  produced  from  a  fluid  which  exudes  from  the  outer 
wood  of  several  species  of  tropical  trees  and  shrubs.  Arti- 
ficial, or  synthetic,  rubber  has  been  made  as  a  laboratory 
experiment,  but  up  to  the  present  time  the  cost  of  making 
artificial  rubber  is  far  in  excess  of  the  cost  of  "natural" 
rubber.  The  rubber  industry  is  well  developed  in  Central 
America  and  the  tropical  countries  of  South  America,  es- 
pecially Brazil,  and  in  Ceylon,  the  Malay  Archipelago, 
the  Dutch  East  Indies,  and  central  Africa. 

The  fluid  from  which  rubber  is  made  exudes  from  a  special 
system  of  ducts  in  the  outer  wood  of  the  rubber-producing 
tree  or  shrub,  and  is  quite  distinct  from  the  ordinary  "sap" 
of  the  tree.  This  fluid  is  known  as  latex.  The  latex  is 
gathered  in  buckets  and  coagulated  into  a  solid  mass  by 

254 


RUBBER,  LEATHER,  BELTING,  ROPE  255 

heat,  by  the  addition  of  chemicals,  or  by  churning.  This 
mass  after  being  et cured"  or  antisepticized  by  smoking  is 
the  " crude"  rubber  which  is  the  raw  material  for  the  manu- 
facture of  rubber  goods.  The  crude  rubber  is  washed  and 
shredded  by  knives,  then  mixed  with  sulphur  (and  other  in- 
gredients varying  for  the  special  service  the  manufactured 
rubber  is  to  perform)  into  a  "'dough,"  and  this  dough  is 
rolled  into  sheets  or  pressed  into  shapes  and  "vulcanized" 
by  the  combined  action  of  heat  and  pressure.  For  thin 
sheets  the  vulcanizing  may  be  accomplished  by  treating 
the  crude  rubber  sheet  with  a  solution  of  carbon  bisulphide. 
The  percentage  of  sulphur  used  in  vulcanizing  determines 
whether  the  rubber  shall  be  hard  or  soft.  A  high  percent- 
age of  sulphur  gives  hard,  brittle  rubber,  and  a  low  percent- 
age gives  soft  rubber.  For  a  good  grade  of  soft  rubber  a 
combination  of  92.5  per  cent,  crude  rubber  with  7.5  per 
cent,  of  sulphur  is  not  uncommon. 

Physical  Properties  of  Rubber. — The  physical  proper- 
ties of  different  kinds  of  rubber  vary  over  a  wide  range. 
Soft  rubber  will  stretch  from  six  to  ten  times  its  original 
length  without  breaking',  while  very  hard  rubber  is  almost 
as  brittle  as  cast  iron,  though  it  may  be  made  flexible 
by  the  application  of  heat  as  low  as  that  of  boiling  water. 
Fig.  96  gives  stress-strain  diagrams  for  typical  samples 
of  good  quality  soft  rubber,  for  poor  quality  soft  rubber,  and 
for  hard  rubber.  The  wide  difference  between  the  shape 
of  the  stress-strain  diagram  for  soft  rubber  and  the  typical 
stress-strain  diagrams  shown  in  Fig.  14  and  15  is  note  worthy. 
The  reversal  of  curvature  at  the  end  of  the  stress-strain 
diagram  for  rubber  is  especially  worthy  of  note. 

Fig.  97  gives  a  typical  diagram  for  soft  rubber  in  com- 
pression. It  is  to  be  noted  that  this  diagram  is  slightly 
concave  toward  the  stress  axis — the  reverse  of  the  dia- 
grams shown  in  Fig.  14  and  15. 

The  maximum  unit  tensile  stress  carried  by  good  quality 
soft  rubber  varies  from  800  to  1200  Ib.  per  sq.  in.  Poor 
quality  soft  rubber  may  fail  in  tension  at  a  stress  as  low  as 
200  Ib.  per  sq.  in.  In  compression  rubber  shows  no  well- 


256 


MATERIALS  OF  ENGINEERING 


defined   ultimate   strength.     The   safe   working   limit   of 
compression  for  good  quality  rubber  may  be  taken  as  a 


1000 


0          1.0         2.0        3.0       4-.0        5.0       6.0        7.0 
Unit  Strain  Inches  per  Inch  Length. 

FIG.  96. — Stress-strain  diagrams  for  rubber  in  tension. 

compression  to  one-half  the  original  height  of  the  rubber; 
this  corresponds  to  a  stress  of  about  500  to  800  Ib.  per  sq. 


/ 

/ 

c 

Jt 

L 

^-300 

/ 

1 

/ 

Unit  Stress 

8  i 

/ 

/ 

O.I  0.2  0.3  0.4 

Unit  Strain  in  per  Inch  in  Length 


0.5 


FIG.  97. — Stress-strain  diagram  for  soft  rubber  in  compression. 

in.     The  great  flexibility  of  rubber  renders  it  generally 
unsuitable  for  resisting  flexure  or  torsion. 


RUBBER,  LEATHER,  BELTING,  ROPE  257 

Energy  absorbed  by  Rubber  under  Stress. — Volume  for 
volume  or  weight  for  weight  rubber  under  stress  can  absorb 
very  much  more  energy  than  can  steel  or  any  other 
metal.  Taking  the  sample  of  rubber  whose  stress-strain 
diagram  is  shown  in  Fig.  96  at  a  stress  of  200  Ib.  per  sq. 
in  the  extension  is  5.80  inches  per  inch  original  length.  The 
energy  required  to  produce  this  stretch  is  approximately 
0.5  X  200  X  5.80  =  580  inch-pounds  per  cubic  inch  of 
material. 

For  the  sample  whose  stress-strain  diagram  in  compres- 
sion is  shown  in  Fig.  97  the  energy  required  to  compress 
the  rubber  to  one-half  its  original  thickness  is  approxi- 
mately 151.5  inch-pounds  per  cubic  inch  of  material. 
These  values  may  be  compared  with  the  energy  necessary  to 
stress  spring  steel  in  tension  or  compression  up  to  a  working 
stress  of  50,000  Ib.  per  sq.  in.  Taking  E  for  steel  as 
30,000,000  Ib.  per  sq.  in.  the  unit  strain  corresponding 
to  50,000  Ib.  per  sq.  in.  is  0.00167  and  the  energy  required 
to  produce  this  strain  0.5  X  50000  X  0.00167  =  41.7  inch 
pounds  per  cubic  inch  of  material. 

For  structural  steel  stressed  up  to  16,000  Ib.  per  sq.  in.  the 
energy  required  is  4.27  inch-pounds  per  cubic  inch  of 
material.  The  energy  per  cubic  inch  of  material  required 
to  produce  a  safe  working  stress  is  a  measure  of  the  shock — 
absorbing  capacity  of  the  material,  and  the  figures  given 
in  this  paragraph  show  that  for  tension  members  (such  as 
pneumatic  tires)  rubber,  volume  for  volume,  will  absorb 
about  ten  times  as  much  energy  as  spring  steel,  and  for 
compression  members,  such  as  buffers,  rubber,  volume  for 
volume,  will  absorb  about  3.75  times  as  much  energy  as 
will  spring  steel. 

"Mechanical  Hysteresis"  of  Rubber. — In  common 
speech  rubber  is  spoken  of  as  highly  elastic.  Using  the 
technical  definition  of s  elasticity  given  on  p  3  rubber  is 
not  perfectly  elastic.  Moreover,  if  rubber  is  subjected 
to  a  cycle  of  stress  (see  p.  42)  a  considerable  amount  of 
the  energy  required  to  deform  the  rubber  is  lost  in  mechani- 
cal hysteresis.  Fig.  98  gives  stress-strain  diagrams  for 

17 


258 


MATERIALS  OF  ENGINEERING 


cycles  of  stress  for  two  typical  samples  of  rubber.  The 
amount  of  energy  lost  in  mechanical  hysteresis  is  measured 
by  the  shaded  area  for  each  diagram.  The  hysteresis  loss 
is  considerable  even  when  the  rubber  is  loaded  and  un- 
loaded at  a  slow  rate,  and  it  is  much  larger  when  rubber  is 
subjected  to  rapidly  applied  cycles  of  stress.  The  stress- 
strain  diagram  for  rubber  is  greatly  affected  by  the  speed 
of  loading. 

The  energy  lost  in  hysteresis  is  such  members  as  pneu- 
matic tires  on  automobiles  running  at  high  speed  is  sufficient 
to  produce  appreciable  heat,  which  in  itself  tends  to  weaken 
the  rubber.  The  energy  lost  in  mechanical  hysteresis  in 


2000 


5.0  10.0  5.0  10.0 

Unit  Strain  in  per  Inch  Length 

FIG.  98. — Stress-strain  diagrams  for  rubber  subjected  to  a  cycle  of  stress. 

rubber  tires,  rubber  belting  and  other  rubber  parts  sub- 
jected to  rapid  changes  of  load  may  play  an  important  part 
in  shortening  the  life  of  such  parts. 

Deterioration  of  Rubber. — Under  the  action  of  air  and 
of  light  vulcanized  rubber  tends  to  become  hard  and  brittle 
with  the  lapse  of  time.  This  is  illustrated  by  the  gradual 
loss  of  " stretch"  in  rubber  bands  left  lying  on  desks  for 
some  time.  Rubber  is  little  affected  by  dilute  acids,  by 
water,  or  by  dilute  alkalies,  but  is  readily  "rotted"  (ren- 
dered brittle)  by  oils.  The  deterioration  of  rubber  de- 
pends to  a  considerable  extent  to  the  presence  of  foreign 
ingredients,  such  as  chalk  or  zinc  oxide,  which  are  frequent- 
ly mixed  with  rubber  and  sulphur  before  vulcanizing. 
Rubber  is  inflammable  and  melts  at  about  370  degrees 
Fahr.  It  cannot  be  used  at  high  temperatures. 


RUBBER,  LEATHER,  BELTING,  ROPE  259 

Leather.— As  an  engineering  material  leather  is  used  in 
two  forms,  rawhide  and  tanned  leather.  Rawhide  is 
the  salted  hide  of  animals,  usually  ox-hide.  It  is  used  for 
gears,  belt  lacing,  and  for  some  belts,  though  tanned  leather 
is  the  usual  material  for  belting.  Rawhide  is  a  tough 
strong  material  with  little  capacity  for  "  stretch. "  Raw- 
hide gears  have  about  the  strength  of  cast  iron  gears  under 
steady  load,  and  a  higher  strength  under  shock.  Rawhide 
gears  operate  with  very  little  noise. 

Tanned  leather  is  prepared  by  treating  raw  hides  with 
a  tanning  solution  prepared  from  oak  bark.  Leather  is 
used  for  belting  and  for  hydraulic  packings.  Leather 
suitable  for  good  quality  belting  is  obtained  from  the  part 
of  hides  left  after  cutting  away  the  belly.  Belting  is  built 
up  by  cementing  or  riveting  together  strips  of  tanned  lea- 
ther. Single  ply  belting  is  made  from  one  thickness  of 
leather,  two-ply  belting  from  two  thicknesses  of  leather 
and  so  on.  Single  ply  leather  belting  is  about  0.23  inches 
thick  and  two-ply  leather  belting  is  about  0.34  inches 
thick. 

Weight  and  Strength  of  Leather  Belting. — Leather 
weighs  about  0.035  Ib.  per  cu.  in.,  its  specific  gravity  is 
very  nearly  unity.  The  ultimate  tensile  strength  of  good 
quality  leather  belting  is  about  3800  Ib.  per  sq.  in.,  which 
corresponds  to  a  strength  of  about  900  Ib.  per  inch  width 
for  single  ply  belting  and  about  1300  Ib.  per  inch  width  for 
two-ply  belting.  Under  a  tensile  load  of  2250  Ib.  per  sq. 
inch  applied  for  an  hour  belting  should  stretch  not  more 
than  13.5  per  cent,  of  its  original  length. 

Strength  of  Belt  Joints. — If  the  free  ends  of  a  piece  of 
belting  are  fastened  together  by  chamfering,  lapping, 
and  cementing  the  strength  of  the  joint  can  be  made  nearly 
equal  to  that  of  the  leather.  Joints  in  belting  are,  however 
more  commonly  made  by  lacing  the  free  ends  together  with 
rawhide  strips,  with  wire,  or  by  using  some  special  form  of 
flexible  metal  connection.  The  best  joints  (with  the 
exception  of  cemented  joints)  are  made  by  the  use  of  special 
lacing  machines  which  thread  a  spiral  of  steel  wire  through 


260  MATERIALS  OF  ENGINEERING 

each  free  end  of  the  belt.  The  two  spirals  are  dovetailed 
together,  and  a  pin  of  metal  or  of  raw-hide  is  run  through 
the  two  spirals  making  a  hinge  joint. 

'  Ordinary  laced  joints  in  belting  have  about  one-third 
the  strength  of  leather;  joints  made  by  the  use  of  special 
lacing  machines  may  have  strength  as  high  as  one-half  the 
strength  of  the  leather. 

Canvas  Belting,  Rubber  Belting. — Woven  canvas  belting 
is  made  in  four,  six,  eight,  and  ten-ply  thicknesses.  It 
weighs  from  0.03  Ib.  per  cu.  in.  to  0.05  Ib.  per  cu.  in.  de- 
pending on  the  waterproofing  and  sizing  material  used.  It 
is  about  as  strong  as  leather  belting,  but  has  not  so  much 
" stretch'7  nor  so  high  a  coefficient  of  friction  between 
itself  and  the  surface  of  pulleys  as  leather  belting.  It  is 
used  mainly  for  agricultural  machinery. 

Rubber  belting  is  made  on  a  foundation  of  woven  duck, 
impregnated  with  rubber.  Its  special  feature  is  its  re- 
sistance to  moisture,  and  it  us  used  for  driving  machinery 
in  damp  locations.  Rubber  belting  weighs  about  0.045 
Ib.  per  cu.  in.  and  has  an  ultimate  tensile  strength  of  about 
900  Ib.  per  sq.  in. 

Rope. — Rope  is  made  by  straightening  and  twisting 
together  the  fibers  of  certain  plants,  especially  the  fibers 
of  hemp;  it  is  also  made  from  cotton  yarn.  Manila  hemp 
rope  has  a  weight  w,  measured  in  pounds  per  foot  of  about 

w  =  0.32  d2 

in  which  d  is  the  diameter  of  the  rope  in  inches.  The 
ultimate  tensile  strength  (T.S.)  of  good  quality  Manila 
hemp  rope  is  given  by  the  equation 

T.S.  =  100  d2(81-9d) 

For  rope  made  from  cotton  yarn  the  equation  for  weight 
in  pounds  per  foot  is 

w  =  0.26  a2 

and  the  equation  for  tensile  strength  is 
T.  S.    =  4600  d2 


RUBBER,  LEATHER,  BELTING,  ROPE  261 


Selected  References  for  Further  Study 

SCHIDROWITZ:  "Rubber,"  London,  1911.     A  comprehensive  treatise  on  the 

manufacture,   properties,   and  uses   of  rubber  by   a  leading   British 

authority. 
"Rope  and  Rope-making."     A  trade  publication  by  the  Waterbury  Rope 

Company,  New  York. 
"The  Story  of  Schiren  Building."     A  trade  publication  by  the  Charles  A. 

Schieren  Company,  New  York. 


CHAPTER  XX 
TESTING  AND  INSPECTION;  TESTING  MACHINES 

Growing  Importance  of  Testing. — In  the  earlier  days  of 
the  steel  industry — or  of  any  of  the  industries  for  the  pro- 
duction of  materials  of  construction — the  consumer  in  de- 
ciding from  whom  to  buy  his  material  depended  mainly  on 
the  reputation  of  the  producer.  Some  well-known  "brand" 
of  steel  or  cement  was  purchased  in  order  to  insure  good 
material.  As  the  industries  developed  and  the  products  of 
manufacturers  become  standardized,  it  became  necessary 
to  establish  standards  of  quality  for  materials,  and  to  devise 
tests  which  should  determine  the  acceptability  or  non-ac- 
ceptability of  any  shipment  of  material.  The  significance 
of  the  trade-mark  decreased,  while  that  of  the  testing 
laboratory  report  increased.  Today  the  systematic  test- 
ing of  quality  has  become  a  recognized  part — small,  but 
important — of  the  system  of  manufacturing  and  marketing 
the  materials  used  for  machines  and  structures. 

The  use  of  testing  as  a  basis  for  acceptance  of  shipments 
of  materials  is  very  common  today,  and  is  becoming  more 
and  more  common  every  year.  The  use  of  test  results  as  a 
criterion  for  acceptability  renders  possible  the  use  of  a 
selected  quality  of  certain  kinds  of  material,  which,  if  not 
submitted  to  test,  would  not  be  trustworthy  for  structural 
use.  An  illustration  of  this  is  furnished  by  the  use  of  rein- 
forcing rods  for  concrete  which  are  made  from  re-rolled 
rails.  Some  shipments  of  such  re-rolled  rods  are  not  of 
good  quality,  and  unless  samples  from  a  shipment  are 
tested,  it  is  not  safe  to  use  such  re-rolled  material.  If 
samples  from  a  shipment  are  tested  and  the  test  results 
show  strong,  non-brittle  material,  it  is  safe  to  use  such 
material. 

262 


TESTING  AND  INSPECTION:  TESTING  MACHINES  263 

In  general,  tests  furnish  a  better  criterion  of  quality  of 
material  than  is  afforded  by  the  general  reputation  of  the 
manufacturer. 

The  Testing  Engineer. — As  the  practice  of  judging  the 
acceptability  of  shipments  of  materials  from  the  results  of 
tests  becomes  more  and  more  the  general  rule,  the  work 
of  the  testing  engineer  who  plans  and  conducts  such  tests  be- 
comes of  increasing  importance.  His  service  to  the  public 
is  no  small  service;  he  safeguards  buildings,  bridges,  ships, 
machines,  and  roads  against  danger  of  failure  on  account  of 
poor  material,  he  makes  possible  the  use  of  new  materials, 
and  widens  the  field  of  use  of  well-known  materials.  The 
testing  engineer  should  possess  the  highest  integrity,  and 
should  have  a  clear  understanding  of  the  mechanics  of 
materials  and  of  the  general  properties  of  known  materials. 
He  must  not  only  be  proof  against  any  outside  influence 
tending  to  cause  him  to  report  dishonest  results,  but  also 
against  self-deception  and  pre judgment  as  to  the  out- 
come of  tests.  Having  made  tests  carefully,  he  must 
possess  the  courage  to  stand  by  the  results  of  the  tests, 
whatever  those  results  may  be.  He  should  exercise  tact 
and  sound  judgment  in  interpreting  the  results  of  tests,  and 
in  order  that  he  may  do  so,  he  should  have  a  clear  under- 
standing of  both  the  content  of  and  the  reasons  underlying 
the  codes  of  standards  for  materials,  and  the  methods  of 
testing  used. 

Definition  of  Terms. — Inspection  of  materials  of  construc- 
tion comprises  the  examination  for  surface  defects,  for  cor- 
rectness of  dimensions,  for  methods  of  manufacture,  etc., 
and  also  the  making  of  tests  to  see  whether  materials  pos- 
sess the  required  qualities.  Testing  includes  the  making  of 
standard  tests  to  determine  whether  materials  possess 
required  qualities,  and  also  the  making  of  special  tests  of 
materials  to  determine  properties  not  thoroughly  known. 
Tests  of  material  comprise  chemical  analyses,  tests  of 
strength,  hardness,  toughness,  and  ductility,  and  micro- 
scopic examination.  The  statement  of  requirements  as  to 
correctness  of  dimension,  surface  finish,  strength,  chemical 


264  MATERIALS  OF  ENGINEERING 

ingredients,  freedom  from  defects  of  structure,  etc.,  forms 
the  specification  which  samples  taken  from  a  shipment  of 
that  material  must  "pass." 

Commercial  Testing. — Commercial  testing  consists,  in 
general,  in  making  tests  on  selected  samples  from  a  ship- 
ment of  material.  It  is  evident  that  the  proper  selection 
of  samples  is  of  very  great  importance.  The  samples 
should  be  taken  from  various  parts  of  the  shipment,  and 
all  samples  should  be  so  marked  as  to  make  identification 
easy  and  certain.  Carelessness  or  lack  of  thoroughness  in 
sampling  is  one  of  the  most  serious  sources  of  trouble  in 
commercial  testing. 

On  the  result  of  commercial  tests  depends  the  acceptance 
or  rejection  of  large  shipments  of  material,  and  the  tests 
should  be  made  with  a  high  degree  of  precision.  Commer- 
cial testing  must  also  be  rapid.  The  methods  and  appara- 
tus used  should  be  of  the  simplest  character  consistent 
with  accuracy  of  work. 

Chemical  tests  are  very  commonly  used  in  commercial 
testing.  In  general,  chemical  tests  are  made  to  determine 
the  presence  of  a  sufficient  amount  of  a  desired  ingredient 
(e.g.,  carbon  in  rail  steel)  or  to  determine  the  absence  of 
dangerous  amount  of  an  undesirable  ingredient  (e.g.,  sul- 
phuric acid  in  Portland  cement  or  phosphorus  in  steel). 
A  chemical  analysis  does  not  give  complete  information  as 
to  the  nature  and  properties  of  a  material.  Two  pieces  of 
steel  may  show  the  same  chemical  composition,  but  in  a 
testing  machine  may  develop  widely  different  strength. 

Microscopic  examination  of  the  structure  of  a  material 
is  used  in  connection  with  commercial  tests  of  materials, 
especially  of  metals.  Microscopic  examination  reveals  the 
structure  and  the  texture  of  the  material,  and  sometimes 
may  be  used  to  detect  the  presence  of  flaws.  Microscopic 
tests  are  used  as  auxiliary  to  tests  of  strength  and  chemical 
tests  to  furnish  additional  experimental  evidence  on  the 
structure  of  the  material,  rather  than  as  the  main  tests 
to  determine  acceptability  or  non-acceptability. 

Physical  tests  of  samples  are  very  commonly  used  to 


TESTING  AND  INSPECTION:  TESTING  MACHINES  265 

determine  the  acceptability  or  non-acceptability  of  a  ship- 
ment of  material.  The  commonest  strength  test  is  a  ten- 
sion test  to  destruction.  Such  tests  are  made  in  some  form 
of  testing  machines. 


^.Spherical  Bearing, 

Head  §3 
Testing 
Machine     ' 


Cross  Head 
of  Testing 
„..  Machine 

Spherical  Bearing'' 
Threaded    Ends  Shoulder 


1 


Solid 
Ian 

Rm'9      Wedge 
5plitSocket    6riP5 


Wedge 


Ends 


SHORT    TENSION     SPECIMENS 


TENSION  SPECIMEN 
FOR  ROD   OR   WIRE 


Upper  Head 

of  Testing 

Machine 


f 


Cross  Head 

of  Testing 

Machine 


TENSION  SPECIMEN 
FOR  PLATE  MATERIAL 
(Held  in  Wedge  Grips) 


.Leaded  Joint 


Leaded  Joint* 


TENSION   SPECIMEN    FOP  WIRE 
Cross-Head  of     ROPE 
Machine 


Testing 
'//////y 


Weighing  Table 
of  Testing  Machr* 


CYLINDRICAL  COMPRESSION 

SPECIMEN          n 
For  Cement  D-2 
for  Concrete  D-6"to  8" 


TENSION  SPECIMEN  (BRIQUETTE) 

(For  Cement) 
dr/cjuettt  is  I  "x  I"  at  smallest- 
Cross -Section 


Cross -Head  of 
Testing    Machine 


k- 6"- >j< £*•—> 

CROSS- BENDING  TEST  SPECIMEN 
(For  Cast  Iron) 


FIG.  99. — Various  forms  of  test  specimens  for  tests  of  strength  of  materials. 


The  properties  of  materials  commonly  determined  in 
commercial  tests  of  specimens  in  a  testing  machine  are: 
(1)  the  yield  point  (in  some  cases  the  proportional  limit), 


266  MATERIALS  OF  ENGINEERING 

(2)  the  ultimate  strength,  and  (3)  the  elongation  of  the 
test  specimen  after  rupture.  For  brittle  materials  the 
determination  of  the  yield  point  is  not  made.  Fig.  99 
shows  forms  of  test  pieces  in  common  use  for  strength  tests 
of  various  materials. 

Load  tests/  not  to  destruction,  are  occasionally  used 
to  determine  the  acceptability  of  car  couplers  and  some 
other  machine  or  structural  elements  and  also  for  completed 
bridges,  and  for  floors  of  buildings.  In  such  service  tests 
a  load,  called  a  proof  load,  is  applied.  This  proof  load  is 
somewhat  greater  than  the  working  load  for  the  member  or 
structure,  and  under  such  test  load  an  examination  is  made 
to  detect  evidence  of  structural  damage,  such  as  undue 
deformation,  flaking  off  of  paint  or  scale,  cracks,  etc. 

Impact  tests  are  made  on  car  couplers,  rails,  and  some 
other  members  used  in  railway  service.  For  such  tests  a 
known  weight  is  allowed  to  fall  through  a  given  height 
striking  the  sample  piece  to  be  tested.  The  acceptability 
of  the  shipment  is  judged  by  the  amount  of  permanent  dis- 
tortion, cracking,  etc.,  developed  by  the  blow. 

Research  Testing. — Research  testing  includes  tests  made 
to  determine  the  properties  of  materials  whose  general 
qualities  aie  not  well  known.  Mechanical  research  tests 
•include  not  only  tension  tests,  but  also  compression,  flexural 
and  torsion  tests;  tests  under  impact  load;  endurance 
tests  under  repeated  stress;  wear  tests  under  abrasion; 
and  hardness  tests.  In  addition  to  the  study  of  properties 
of  little-known  materials  research  tests  include  tests  made 
with  the  object  of  determining  the  form  of  specimen,  kind 
of  test,  and  procedure  in  testing  best  suited  for  commercial 
tests  of  materials. 

Research  testing  does  not  necessarily  involve  more 
delicate  or  more  accurate  apparatus  and  manipulation 
than  commercial  testing.  In  commercial  testing  many 
arbitrary  factors  are  present,  such  as  size  and  shape  of 
specimen,  speed  of  testing,  type  of  apparatus  used,  etc.; 
the  important  consideration  is  that  a  commercial  test  should 
be  made  under  standard  conditions.  In  research  testing 


TESTING  AND  INSPECTION:  TESTING  MACHINES  267 

it  is  necessary  to  investigate  the  effect  of  varying  conditions 
of  testing  on  the  results  of  the  tests,  and  to  determine 
properties  of  material  as  nearly  independent  of  arbitrary 
test  conditions  as  is  possible. 

Testing  Machines.—  Tension-  Compression-  Flexure 
Machines. — The  type  of  testing  machine  in  most  general 
use  in  the  United  States  can  be  used  for  tests  of  specimens 
in  tension,  in  compression,  or  in  flexure  (or  cross-bending). 


UPPER 
HEAD 


"  s 

FIG.   100. — Diagram  of  screw-power  testing  machine. 

Fig.  100  shows,  in  diagram,  the  arrangement  of  parts  of 
such  a  testing  machine.  Power  is  supplied  through  belt- 
driven  pulleys,  or  from  a  direct-connected  motor,  to  a 
drive  shaft.  The  power  is  transmitted  through  gearing  to 
the  main  screws  S.  By  means  of  the  gearing  and  the  screws 
a  slow  motion  is  given  to  the  cross-head  of  the  testing 
machine.  The  use  of  reducing  gearing  and  screws  greatly 
magnifies  the  force  applied  at  the  drive  pulleys.  As  shown 


268  MATERIALS  OF  ENGINEERING 

in  Fig.  100,  the  machine  is  rigged  for  a  tension  test  and  the 
lower  end  of  the  specimen  Sp  is  held  in  a  socket,  which  is 
attached  to  the  cross-head;  the  upper  end  of  the  specimen 
is  held  in  a  socket  which  is  attached  to  the  upper  head  of  the 
testing  machine.  As  the  cross-head  moves  downward,  the 
specimen  is  put  in  tension,  and  a  downward  force  is  trans- 
mitted by  the  side  struts  T,  to  the  weighing  table  of  the 
machine.  The  weighing  table  rests  on  the  knife  edges  of  a 
pair  of  compound  levers  LiZ//  which  transmit  the  force 
(reduced)  to  the  intermediate  lever  L2,  which,  in  turn, 
transmits  the  force  (still  further  reduced)  to  the  weighing 
beam  which  is  kept  in  balance  by  moving  the  poise.  It 
should  be  noted  for  this  type  of  machine  the  position  of  the 
poise  on  the  weighing  beams  gives  the  load  on  the  specimen 
if  the  beam  is  in  balance  but  not  otherwise. 

In  making  tension  tests  it  is  of  great  importance  that 
the  device  used  for  gripping  the  test  specimen  shall  cause 
an  axial  load  on  the  specimen,  and  that  the  stress  shall  be 
uniformly  distributed  over  the  cross-section  of  the  specimen. 
The  upper  row  of  diagrams  in  Fig.  99  shows  types  of  ten- 
sion grips  in  common  use.  The  use  of  a  gripping  device 
with  spherical  seat  is  desirable  whenever  feasible.  The 
upper  part  of  Fig.  99  shows  the  common  forms  of  tension 
test  specimen.  It  must  be  recognized,  of  course,  that  no 
gripping  device  will  give  an  absolutely  uniform  stress 
distribution  in  the  specimen;  the  devices  shown  give  satis- 
factory results  if  used  carefully. 

The  lower  left  hand  diagram  of  Fig.  99  shows  the  ar- 
rangement of  a  compression  test  specimen  in  a  testing 
machine.  The  test  specimen  is  shown  fitted  with  a  spher- 
ical seated  bearing  block,  which  should  always  be  used 
for  a  compression  test.  The  ends  of  a  compression  test 
specimen  should  be  machined  to  a  plane  surface  for  metal 
or  wooden  specimens  or  the  ends  should  be  made  plane  by 
the  use  of  plaster  of  paris  and  a  flat  piece  of  plate  glass  for 
concrete  or  brick  specimens.  The  best  form  of  compression 
test  specimen  is  a  circular  cylinder,  though  for  convenience 
square  specimens  are  usually  used  for  wood,  and  brick  or 


TESTING  AND  INSPECTION:  TESTING  MACHINES  269 


terra  cotta  blocks  are  usually  tested  in  the  form  in  which 
they  are  produced. 

The  lower  right  hand  diagram  of  Fig.  99  shows  a  small 
cross-bending  specimen  in  a  testing  machine,  and  Fig.  101 
shows  the  arrangement  of  a  testing  machine  for  testing 
a  large  beam.  Large  testing  machines  are  frequently 
equipped  with  a  special  long  weighing  table  for  flexure 
tests  of  large  beams.  In  the  flexure  test  shown  in  Fig.  99 
the  testing  machine  applies  a  concentrated  load  at  mid-span 
of  the  specimen,  while  in  the  flexure  test  shown  Fig.  101 
two  loads  symmetrically  spaced  with  reference  to  mid- 
span  are  applied  to  the  specimen. 


Auxiliary  Beam 


—  Cross-head 

Spherical  Bearing 
Test  Specimen 
..-Roller  Bearing 


FIG.   101. — Screw-power  testing  machine  rigged  for  test  of  beam. 

For  the  specimen  with  two  equal  loads  symmetrically 
spaced  the  shear  between  the  loads  is  zero,  and  the  bending 
moment  for  any  point  between  them  is  equal  to  Pa  (see 
Fig.  5).  This  arrangement  of  loading  puts  a  consider- 
able length  of  specimen  under  a  constant  bending  moment, 
whereas  with  the  loading  shown  in  Fig.  99  only  the  cross- 
section  at  mid-span  is  subjected  to  the  maximum  bending 
moment.  The  two-load  text  subjects  a  greater  portion 
of  the  specimen  to  maximum  fiber  stress  than  does  the 
one-load  test. 

Fig.  100  shows  the  type  of  testing  machine  in  commonest 
use  in  the  United  States.  Another  type  which  is  used  is 
shown  in  Fig.  102.  This  type  is  common  for  very  large 


270 


MATERIALS  OF  ENGINEERING 


machines.  Hydraulic  pressure  is  supplied  from  a  pump  or 
an  accumulator  through  the  pipe  W  and  forces  the  piston 
P  and  the  bearing  block  B  against  the  specimen  S.  The 
specimen  bears  against  the  upper  head  U  which  is  adjust- 
able for  position,  being  moved  up  or  down  by  the  motor 
M  and  suitable  gearing.  The  pressure  is  transmitted  to 
vertical  threaded  rods  T  through  nuts  N,  and  thence 


FIG.  102. — Hydraulic  press  type  of  testing  machine.  Machine  shown  is 
adapted  for  compression  tests  only,  this  type  of  machine  can  be  constructed  so 
as  to  make  tension  tests  and  flexure  tests  also. 

is  transmitted  to  the  lower  plate  L,  which  takes  the  reaction 
of  the  cylinder  C.  The  pressure  is  measured  by  means  of 
a  pressure  gage  G,  or  by  the  pressure  on  a  smaller  piston 
P'  whose  upward  thrust  is  resisted  by  the  weighing  scale 
VK.  Knowing  the  intensity  of  pressure  the  load  on  the 
specimen  is  obtained  by  multiplying  this  intensity  by  the 
area  of  the  piston  P.  Fig.  102  shows  a  machine  arranged 
for  compression  tests  only,  but  machines  of  this  type  are 


TESTING  AND  INSPECTION:  TESTING  MACHINES  271 

also  built  arranged  for  tension  tests.  The  10,000,000-lb. 
compression  testing  machine  of  the  U.  S.  Bureau  of  Stand- 
ards at  Pittsburgh  Pa.,  which  is  the  largest  testing  machine 
in  the  world,  is  of  this  type.  For  large  machines  it  is 
necessary  to  use  leather  or  hemp  packings  on  the  piston 
P  to  secure  a  sufficiently  tight  joint  for  operation  of  the 
machine,  and  the  variation  of  the  friction  of  the  packing 
against  the  piston  may  cause  a  rather  considerable  error 
in  the  indications  of  the  machine.  Small  machines  of 
this  type  are  built  without  packing,  the  pistons  being 
carefully  lapped  to  fit  the  cylinders.  Such  machines  are 


Large  Hydraulic  Support 

Flexible  Copper 

Force- Applying  End  :   Force-Weighing  End      .-Tube 

Diaphragm.  \ 

r™^         '        Diaphragrr, 


For  Compression         /         Small  Hydraulic 
this  Point  Bears        j  Support 

For  Compression  this  Point 
does  not  Bear 
FIG.   103. — Emery  type  of  testing  machine. 

as  accurate  as  the  knife-edge  scale  type  of  machine  shown 
in  Fig.  100.  The  hydraulic  press  type  of  machine,  shown 
in  Fig.  102  is  cheaper  to  build  than  a  knife-edge  scale 
type  of  machine  of  the  same  capacity. 

Fig.  103  shows  in  diagram  the  Emery  type  of  testing 
machine.  In  this  machine  the  axis  of  a  tensile  specimen  or 
a  compressive  specimen  is  usually  horizontal.  The  hy- 
draulic cylinder  at  the  left  is  served  by  a  pump  or  an 
accumulator  and  when  the  piston  is  forced  to  the  left 
tensile  stress  is  applied  to  the  specimen  as  shown;  for  a 
compression  specimen  the  piston  is  forced  to  the  right. 
The  pull  or  thrust  is  transmitted  to  a  hydraulic  support, 
consisting  of  a  short  cylinder  fitting  with  a  disc  fastened 
inside  the  cylinder  by  means  of  a  flexible  diaphragm. 


272  MATERIALS  OF  ENGINEERING 

This  support  is  filled  with  liquid  and  connects  through  a 
small  pipe  to  a  smaller  support  which  transmits  the  load 
on  the  specimen,  reduced,  to  a  weighing  scale.  In  the 
Emery  machine  instead  of  knife  edge  supports  the  scale 
levers  are  hung  on  flexible  steel  springs. 

The  Emery  type  of  machine  is  extremely  sensitive  and 
very  costly.  The  advantages  of  a  horizontal  machine  are 
ease  of  placing  large  specimens  in  place;  the  disadvantages 
are  greater  floor  space  required,  and  the  inveitable  bending 
stresses  set  up  by  their  own  weight  in  long  tension  or  com- 
pression specimens.  Flexure  tests  can  be  made  more  easily 
on  a  vertical  machine  (such  as  the  machines  shown  in  Fig. 
100  and  Fig.  102)  than  on  a  horizontal  machine. 

Testing  machines  for  tension,  compression,  and  cross- 
bending  tests  are  made  combining  different  features  shown 
in  Figs.  100,  102,  and  103.  For  example  machines  are  built 
with  a  hydraulic  cylinder  for  applying  load  and  a  weighing 
scale  for  measuring  the  load.  A  common  British  machine 
uses  a  single  lever  for  weighing  the  load  rather  than  a 
system  of  compound  levers.  Another  type  of  machine 
measures  load  by  the  displacement  of  a  heavy  pendulum, 
in  a  manner  similar  to  that  shown  in  Fig.  104  for  a  torision 
testing  machine.  For  testing  special  " arbitration  bars" 
of  cast  iron  special  small  cross-bending  machines  are  built. 

Torsion  Testing  Machines. — For  testing  the  shearing 
strength  of  material  the  torsion  test  is  the  most  suitable, 
because  that  test  produces  pure  shearing  stress  in  a  round 
specimen.  Torsion  tests  cannot  readily  be  made  on  a 
tension-compression-flexure  testing  machine  such  as  is 
shown  in  Fig.  100  and  a  special  form  of  testing  machine  is 
used,  Fig.  104  shows  one  form  of  torsion  testing  machine. 

Power  is  applied  by  hand  through  the  crank  K  (or 
through  a  drive  pulley)  and  then  through  the  worm  M 
and  gear  G  to  turn  the  chuck  C.  The  specimen  S  is  fast- 
ened in  two  similar  chunks,  one  at  each  end  of  the  specimen, 
by  means  of  self-centering  jaws  J.  As  the  specimen  is 
twisted  it  swings  the  heavy  pendulum  P  out  toward  the 
position  P',  and  the  amount  of  twisting  moment  exerted 


TESTING  AND  INSPECTION:  TESTING  MACHINES  273 

by  and  transmitted  through  the  specimen  is  equal  to  Wa: 
W  is  the  weight  of  the  pendulum  and  a  is  the  horizontal 
motion  of  its  center  of  gravity.  Attached  to  the  pendulum 
is  a  finger  T  which  shoves  an  indicator  /  along  a  scale  E. 
The  amount  of  motion  of  the  indicator  over  the  scale  (a') 
is  proportional  to  a  the  horizontal  motion  of  the  center  of 
gravity  of  the  pendulum.  Hence  the  scale  E  can  be 
graduated  to  read  directly  the  amount  of  twisting  moment 
on  the  specimen. 


J 


FIG.   104. — Testing  machine  for  torsion  testa. 

Torsion  test  specimens  are   practically  always  circular 
in  cross-section,  either  solid  or  hollow.     In  addition  to 
type  shown  in  Fig.  104  torsion  testing  machines  are  built 
in  which  the  twisting  moment  is  weighed  by  the  use  of  a 
compound  lever  system  and  a  weighing  scale. 

Measurement  of  Strain,  Extensometers. — In  most  com- 
mercial tests  there  is  no  attempt  to  measure  small  strains 
in  the  specimen.  For  determining  the  ultimate  tensile 
strength  it  is  necessary  merely  to  note  the  maximum  load 
carried  by  the  test  specimen,  to  determine  the  yield  point 
it  is  necessary  merely  to  note  the  load  at  which  the  beam 
of  the  testing  machine  " drops"  and  stays  down  for  a  second 


274 


MATERIALS  OF  ENGINEERING 


or  two  as  the  cross-head  moves  downward  at  a  uniform 
rate,  or  to  note  the  load  when  visible  stretch  of  the  specimen 
can  be  detected  by  the  use  of  a  pair  of  dividers.  The 
elongation  after  fracture  is  measured  directly,  and  the 


(c)  (d) 

FIG.  105. — Various  types  of  extensometers. 

reduced  section  at  fracture  is  measured  by  the  use  of  a 
micrometer  caliper,  as  is  the  original  cross-section. 

When,  however,  it  is  necessary  to  determine  the  modulus 
of  elasticity,  the  elastic  limit  (see  p.  31)  or  the  proportional 
limit  it  becomes  necessary  to  use  some  apparatus  for  meas- 


TESTING  AND  INSPECTION:  TESTING  MACHINES  275 

uring  small  strains.  In  general  the  apparatus  used  should 
be  sufficiently  sensitive  to  detect  a  'change  of  dimension  as 
small  as  0.0001  inch.  Fig.  105  shows  several  types  of 
extensometers  for  measuring  small  strains  in  tension  test 
specimens. 

Fig.  105a  shows  an  extensometer  utilizing  the  principle 
of  the  screw  micrometer.  Clamps  F  are  attached  to  the 
specimen  S  at  the  end  of  the  gage  length,  each  clamp  by 
three  pointed  screws  P.  Micrometer  screws  M  extend 
through  the  lower  clamp  and  may  be  turned  until  their 
points  come  into  contact  with  the  side  rods  K  which  are 
held  in  the  upper  clamp  by  insulating  bushings  R.  The 
contact  of  each  micrometer  screw  and  its  side  rod  is  deter- 
mined by  the  establishment  of  electric  contact  through  a 
circuit  containing  a  telephone  receiver  (a  small  electric 
lamp  or  an  electric  bell  is  sometimes  used).  The  differ- 
ence between  successive  readings  of  the  pointer  /  on  the 
scale  E  of  a  micrometer  gives  the  elongation  along  its 
axis,  and  the  mean  of  the  elongations  shown  by  the  two 
micrometers  gives  the  average  stretch  of  the  specimen. 
In  any  accurate  extensometer  it  is  essential  to  determine 
the  average  stretch  of  the  specimen. 

In  skillful  hands  the  micrometer  type  of  extensometer 
gives  accurate  results,  but  is  is  slow  in  use,  and  any  but 
the  most  delicate  handling  is  apt  to  disturb  its  attachment 
to  the  specimen. 

Fig.  1056,  shows  an  extensometer  in  which  a  microscope 
is  used  to  measure  the  strains  in  the  specimen.  Clamps 
F  are  attached  to  the  specimen  S  at  the  end  of  the  gage 
length,  each  clamp  by  two  pointed  screws  P.  At  the  left 
hand  the  clamps  are  held  a  constant  distance  apart  by  the 
pivot  bar  £7,  which  can  turn  about  the  axis  MZ  and  whose 
upper  end  is  a  sharp  point  which  bears  on  a  conical  bearing 
at  J.  As  the  specimen  stretches  the  right  hand  pivot  J' 
moves  a  distance  equal  to  twice  the  average  stretch  of  the 
specimen,  and  with  the  pivot  J'  moves  the  bar  U'  carrying 
a  small  glass  plate  X  on  which  is  a  horizontal  scratch  made 
by  a  fine  diamond  point,  The  bar  U'  is  guided  in  a  straight 


276  MATERIALS  OF  ENGINEERING 

path  by  the  guide  plate  D.  To  the  lower  clamp  is  attached 
a  microscope  M  fitted  with  an  optical  micrometer  in  the 
eyepiece  Y,  and  the  motion  of  the  scratch  on  the  plate  X 
an  consequently  the  stretch  of  the  specimen  is  measured  by 
means  of  this  eyepiece  micrometer. 

The  microscope  gives  direct  readings  without  the  neces- 
sity of  manipulation,  the  long-continued  use  of  a  micro- 
scope is,  however,  somewhat  wearying  to  the  untrained 
eye.  This  type  of  attachment  of  extensometer  to  specimen 
gives  a  mechanical  average  of  the  stretch  on  the  two  oppo- 
site elements  where  the  pointed  screws  P  are  attached. 

Fig.  105c  shows  an  extensometer  in  which  the  measuring 
unit  is  a  micrometer  dial  gage  which  magnifies  small 
motions  by  means  of  clockwork  gears.  A  number  of  such 
gages  are  on  the  market.  The  attachment  to  the  specimen 
of  the  extensometer  shown  in  Fig.  105c  is  a  modification 
of  the  attachment  shown  in  Fig.  1056.  The  extensometer 
shown  in  Fig.  105c  gives  direct  readings  without  manipu- 
lation, and  gives  directly  the  average  stretch  of  the  speci- 
men. Usually  the  clockwork  dial  is  more  convenient  but 
not  quite  so  accurate  as  the  micrometer  screw  (in  skilful 
hands)  or  the  microscope. 

Fig.  105d  shows  an  extensometer  in  which  the  measure- 
ment of  strain  is  made  by  the  use  of  the  " optical  lever." 
Clamps  F  are  pressed  against  the  specimen  by  means  of 
springs  G.  The  upper  end  of  each  clamp  bears  against  the 
specimen  through  a  sharp  knife  edge  K ;  at  the  lower  end 
of  each  clamp  is  a  small  steel  lozenge  Z  to  which  is  attached 
a  mirror  M.  As  the  specimen  stretches  the  lozenges  Z 
rotate  through  a  very  small  angle,  an  with  them  move 
the  mirrors  M.  The  motion  of  the  mirrors  is  measured 
by  means  of  telescopes  T  and  scales  S,  the  scale  division 
reflectd  into  the  telescope  by  the  mirror  changing  as  the 
mirror  rotates.  This  " mirror  type"  of  extensometer 
can  be  made  to  give  indications  of  very  small  strains  by 
increasing  the  distance  of  the  telescopes  and  scales  from 
the  specimen.  It  is,  in  general,  very  slow  to  operate. 

It  will  be  noted  that  all  the  above  extensometers  are 


TESTING  AND  INSPECTION:  TESTING  MACHINES  277 

arranged  to  give  either  the  average  stretch  of  the  speci- 
men, or  to  give  the  stretch  along  two  symmetrical  gage 
lines,  the  average  of  the  readings  along  these  two  lines 
giving  the  average  stretch  of  the  specimen.  It  is  very 
important  that  extensometers  should  be  arranged  to  give 
average  stretch,  especially  when  used  to  determine  the 
modulus  of  elasticity.  Even  with  the  greatest  care  in 
adjustment  the  stress  across  the  cross-section  of  a  specimen 
is  never  uniform,  and  determinations  of  stretch  along 
any  one  gage  line  are  almost  certain  to  differ  from  the  aver- 
age stretch.  In  very  accurate  determinations  the  stretch 
is  sometimes  measured  along  three  symmetrical  gage 
lines. 

Various  other  forms  of  extensometer  are  in  use,  but  the 
above  examples  are  believed  to  be  typical.  Compresso- 
meters,  deflectometers,  and  torsion  indicators  using  simi- 
lar measuring  units  are  use  in  compiession  tests,  flexure 
tests,  and  torsion  tests.  They  will  not  be  taken  up  in  de- 
tail here. 

Determination  of  the  "Elastic  Limit." — As  noted  on 
p.  32  the  term  elastic  limit  is  used  in  practice  rather 
loosely,  and  its  precise  meaning  is  not  always  clear.  It  is 
determined  in  different  ways  by  different  laboratories, 
and  its  value  depends  to  some  extent  on  its  method  of 
determination.  The  following  is  an  outline  of  several 
methods  in  use  for  determining  elastic  limit,  proportional 
limit,  yield  point,  and  other  related  values.  In  reporting 
a  test  the  method  used  for  determining  elastic  limit,  pro- 
portional limit,  or  yield  point  should  be  indicated. 

Elastic  limit  as  defined  in  "Standard  Methods  of  Testing"  of  the 
1918  A.  S.  T.  M.  Standards,  p.  759,  is:  "the  greatest  load  per  unit  of 
original  cross-section  which  does  not  produce  a  permanent  set.  (See  also 
p.  31.) 

Determination. — The  determination  of  this  limit  involves  the  appli- 
cation and  removal  of  a  series  of  increasing  loads,  with  a  measurement 
of  set  after  each  one,  and  the  plotting  of  sets  as  abscissas  and  loads 
causing  set  (or  unit  stress  corresponding)  as  ordinates,  see  Fig.  10@a. 
The  detecting  of  a  set  depends  on  the  sensitiveness  of  extensometer 
used.  The  A.  S.  T.  M.  Standards  (p.  765)  specify  that  the  extensom- 


278 


MATERIALS  OF  ENGINEERING 


eter  shall  have  a  sensitiveness  of  0.0001  in.  A  specification  of  sensitive- 
ness per  inch  of  gage  length  would  be  better.  Fig.  la  shows  the  method 
of  determining  the  elastic  limit  (denoted  by  E)  from  a  diagram  of 
loads  and  sets  using  0.0001  in.  as  the  standard  of  appreciable  set. 

Commercial  Determination. — This  is  rarely  made,  but  sometimes  is 
made  as  a  proof  test,  to  make  sure  that  set  has  not  occurred  below  in 
certain  stress.  In  this  case  appreciable  set  is  taken  as  set  which  can 
be  measured  directly  by  the  use  of  a  pair  of  dividers,  which  means  a 
sensitiveness  of  about  0.01  in. 

Proportional  limit  is  the  load  per  unit  of  original  cross-section  at 
which  deformations  cease  to  be  directly  proportional  to  the  loads. 


.(Useful  /  \ 


-t  ("Elastic"  Limit) 

(Proportional  Limit) 


O.OOOI- 


~     ./ncj=mn 
/ncj =0.5mn\    I  /  o'cj'is 
.Yo'cj1  is  part_f/   parallel  to 
Yalle/  to  Ocj.  ti/  Oa 


0  000 

->5et  after 

<—  -Removal  of— -->  •< Deformation  under  Load ->- 

Load  bed 

a 

FIG.   106. — Various  methods  of  determining  elastic  limit  and  proportional  limit. 

Determination. — This  determination  must  be  made  from  a  plotted 
load-deformation  diagram  for  a  specimen.  The  A.  S.  T.  M.  Standards 
here  also  specify  the  use  of  an  extensometer  with  a  sensitiveness  of 
0.0001  in.  Fig.  1066,  illustrates  the  determination  of  the  proportional 
limit,  using  0.0001  in.  as  the  limit  for  appreciable  deviation  of  the 
stress-strain  diagram  from  a  straight  line. 

Johnson's  Apparent  Elastic  Limit. — Proposed  by  the  late  J.  B.  John- 
son. This  limit  is  taken  as  that  stress  at  which  the  rate  of  deformation 
isj50  per  cent,  greater  than  the  initial  rate. 

Determination. — This  limit  requires  a  stress-strain  diagram  for  its 
determination.  Fig.  106c  illustrates  such  a  determination.  The 


TESTING  AND  INSPECTION:  TESTING  MACHINES  279 

initial  rate  of  deformation  is  given  by  the  ratio  wn:0w.  nq  is  0.5  of 
mn  so  that  mq  is  1.50  times  mn,  and  the  slope  of  Qq  represents  a  rate  of 
deformation  50  per  cent,  greater  than  the  initial  rate.  Q'q'  is  drawn 
parallel  to  Oq  and  tangent  to  the  stress-strain  diagram.  The  point  of 
tangency  /  locates  Johnson's  apparent  elastic  limit. 

The  Useful  Limit. — Proposed  by  the  column  committee  of  the  A.  S. 
C.  E.  This  limit  is  similar  in  nature  to  Johnson's,  but  it  is  located  at 
that  stress  for  which  the  rate  of  deformation  is  100  per  cent,  greater 
than  the  initial  rate. 

Determination  is  similar  to  the  determination  of  Johnson's  elastic 
limit  and  is  illustrated  by  Fig.  106d. 

A.  S.  T.  M.  Elastic  Limit. — This  is  a  method  of  determining  a  so- 
called  elastic  limit  prescribed  in  certain  specifications  for  steel  by  the 
A.  S.  T.  M.  Standards  (see  1918  Standards,  p.  163).  It  is  determined 
by  the  use  of  an  extensometer  sensitive  to  0.0002  in.,  usually  with  a 
2-in.  gage  length.  This  extensometer  is  attached  to  the  specimen,  and 
load  applied  at  a  uniform  rate.  The  load  at  which  the  pointer  of  the 
extensometer  is  seen  to  move  at  an  accelerated  rate  is  taken  as  this 
limit,  which  is  really  a  yield  point,  but  is  called  an  elastic  limit.  The 
determination  of  the  A.  S.  T.  M.  elastic  limit  does  not  necessitate  the 
plotting  or  the  autographic  drawing  of  a  stress-strain  diagram. 

Yield  Point. — This  is  defined  as  the  stress  at  which  deformation  in- 
creases without  any  increase  of  load. 

Determination. — It  is  usually  determined  by  the  drop  of  the  beam 
of  the  testing  machine  as  load  is  applied  at  a  uniform  rate,  or  by  the 
halt  of  the  pointer  of  a  self-indicating  weighing  device.  This  method 
is  unreliable  for  hard  steels,  as  the  "drop"  is  very  uncertain. 

A  second  method  of  determining  the  yield  point  is  to  use  a  pair  of 
divides  spanning  the  distance  between  two  gage  lines  or  prick  punch 
holes,  and  to  locate  the  yield  point  at  the  stress  when  the  first  stretch 
is  visible  to  the  eye  as  load  is  applied  at  a  uniform  rate.  This  method 
is  not  strictly  consistent  with  the  definition  given  above,  but  works 
well  practically.  It  is  rather  more  reliable  than  the  "drop  of  the 
beam"  method,  though  it  would  not  be  reliable  for  steels  with  a  yield 
point  above  100,000  Ib.  per  sq.  in.  (assuming  a  2-in.  gage  length). 

Impact  Tests  and  Impact  Testing  Machines. — Tests 
of  specimens  by  impact  are  occasionally  made  to  deter- 
mine the  toughness  of  the  material  under  test  (see  p.  35 
for  definition  of  toughness).  Impact  tests  determine  the 
amount  of  energy  required  to  fracture  a  specimen  or  to 
stress  it  to  its  elastic  limit,  and  the  results  are  measured  in 
foot-pounds  or  inch  pounds  not  in  pounds  or  pounds  per 
per  square  inch.  No  direct  comparison  can  be  made  be- 


280 


MATERIALS  OF  ENGINEERING 


tween  the  usual  test  results  obtained  from  a  "static" 
test  of  material  and  an  impact  test  of  material;  the  area 
under  a  static  stress-strain  diagram  gives  a  measure  of 
energy  required  for  rupture  and  this  area  may  be  compared 
with  the  results  of  an  impact  test.  The  commonest  impact 
test  is  a  test  in  flexure,  though  impact  tensile  tests  are  also 
made.  Frequently-  in  testing  ductile  metals  in  flexure  the 
specimen  is  notched  to  localize  stretch  and  insure  complete 
rupture:  tests  of  notched  specimens  give  only  compara- 
tive results  and  great  care  must  be  taken  to  have  all  speci- 


FIG.  107. — Pendulum  type  of  testing  machine  for  impact  tests. 

mens  the  same  size,  and  especially  to  have  the  notches 
uniform  in  size  and  shape. 

Fig.  107  shows  in  diagram  a  form  of  impact  testing 
machine  used  for  testings  small  specimens  in  flexure.  A 
heavy  pendulum  P  is  hung  in  ball  bearings  R,  and  counter- 
weighted  so  that  its  center  of  percussion1  is  at  Z  and  its 
center  of  gravity  at  G.  To  its  axis  is  attached  an  arm  T 
which  as  the  pendulum  swings  to  the  right  pushes  an 
indicating,  finger  N  around  the  scale  E.  The  finger  TV  does 
not  swing  back  with  the  pendulum,  but  remains  at  its 
point  of  highest  swing  to  the  right  N".  The  specimen  S 

•  l  For  a  method  of  locating  the  center  of  percussion  of  a  pendulum  see 
Foorman's  "Applied  Mechanics,"  p.  171, 


TESTING  AND  INSPECTION:  TESTING  MACHINES  281 

is  supported  so  that  it  bears  against  supports  K,  and  as  the 
pendulum  falls  the  specimen  is  struck  by  the  center  of 
percussion  of  the  pendulum.  To  make  a  test  the  specimen 
S  is  placed  in  position,  the  pendulum  P  raised  to  the  posi- 
tion shown  by  the  solid  lines,  held  there  by  the  latch  L, 
and  the  angle  initial  of  the  pendulum  read  by  means  of  the 
pointer  N  (hi  some  machines  the  initial  angle  has  a  definite 
fixe<d  value);  the  latch  is  released  the  pendulum  falls, 
striking  the  specimen  S,  fracturing  it,  and  then  the  pen- 
dulum rises  to  some  final  position  G:"  this  position  is 
determined  by  reading  the  final  position  of  the  pointer  at  N." 
The  energy  utilized  in  fracturing  the  specimen  is  equal  to 
Wh  —  Wh,"  in  which  W  is  the  weight  of  the  pendulum, 
h  is  the  vertical  fall  of  its  center  of  gravity,  and  h"  is  the 
vertical  rise  of  its  center  of  gravity  after  rupturing  the  speci- 
men. The  initial  reading  and  the  final  reading  of  the  point- 
er N  give  measures  repsectively  of  the  fall  and  the  rise  of 
the  center  of  gravity  of  the  pendulum. 

Fig.  108  shows  in  diagram  an  impact  testing  machine  in 
which  the  impact  is  supplied  by  a  weight  falling  vertically. 
The  weight  W  is  raised  to  a  predetermined  height  by  means 
of  a  hoist  D  and  a  lifting  magnet  M .  This  height  is  indi- 
cated on  the  scale  E  by  a  pointer.  The  weight  carries  with 
it  a  pencil  P  which  bears  on  the  surface  of  a  drum  R  which 
is  driven  at  a  constant  speed  of  rotation.  The  specimen 
S  receives  the  impact  of  the  falling  weight.  Two  methods 
of  making  impact  tests  with  the  drop-test  type  of  machine 
are  in  use.  In  the  first  the  weight  is  dropped  from  success- 
ively increasing  heights  until  a  permanent  set  in  the  speci- 
men or  an  abnormal  deflection  under  impact  indicates  that 
the  elastic  limit  has  been  reached,  or  until  rupture  occurs. 
In  the  second  method  the  specimen  is  fractured  by  a  single 
blow.  The  pencil  P  traces  a  line  on  a  piece  of  paper  wrap- 
ped round  the  drum  R. 

For  free  fall  this  line  is  a  parabola  as  shown  in  the  upper 
part  of  Fig.  109,  which  is  a  typical  test  diagram.  0  in  Fig. 
109  corresponds  to  the  location  of  the  weight  when  striking 
the  specimen  S  Fig.  108,  and  the  lower  part  of  Fig.  109  shows 


282 


MATERIALS  OF  ENGINEERING 


the  free  fall  after  the  specimen  is  ruptured.  In  rupturing 
the  specimen  kinetic  energy  is  taken  from  the  falling  weight 
and  its  speed  is  reduced;  after  breaking  the  specimen 
another  free  fall  takes  place.  Ordinates  in  Fig.  109  repre- 
sent distance,  and,  since  the  drum  revolves  uniformly, 


FIG.   108.- — Falling-weight  type  of  machine  for  making  impact  tests. 

abscissas  represent  time;  hence,  the  slope  of  the  diagram 
of  Fig.  109  at  any  point  gives  a  measure  of  the  velocity 
of  the  falling  weight  at  that  point.  The  amount  of  energy 
absorbed  in  breaking  the  specimen  can  be  determined  if 
the  velocities  at  two  points  in  the  fall  are  determined,  one 


TESTING  AND  INSPECTION:  TESTING  MACHINES  283 

before  the  weight  strikes  the  specimen  and  one  after  rupture. 
In  Fig.  109  let  the  first  point  be  chosen  at  a  and  the  second 
at  b,  and  let  the  vertical  distance  from  a  to  6  be  denoted 
by  h.  The  velocity  of  the  falling  weight  at  a  is  given 
by  the  slope  of  the  diagram  at  a;  call  this  velocity  va.  Sim- 
ilarly determine  vb,  the  velocity  at  6.  The  kinetic  energy 
of  the  falling  weight  is 

1  W 


FIG.   109. — Diagram  from  test  with  falling-weight  type  of  testing  machine  for 

impact  tests. 

in  which  W  is  the  weight  of  the  falling  weight  and  g  is  the 
acceleration  due  to  gravity  (32.2  ft.  per  sec.  per  sec.). 
If  the  weight  had  fallen  freely  to  b,  the  kinetic  energy  at 
b  would  have  been 


284  MATERIALS  OF  ENGINEERING 

but  the  velocity  at  b  is  actually  vb,  and  the  kinetic  energy 
in  the  falling  weight  at  b  is 

*E  i 

2  g 

The  energy  which  has  been  absorbed  in  breaking  the  test 
specimen  is  then 

1  W  <2 

2  g  l 

In  this  discussion,  friction  of  the  guides  for  the  falling  weight  is 
neglected,  as  is  the  energy  absorbed  in  vibrations  of  specimen,  and 
base  Q,  but  these  losses,  in  general,  are  not  large. 

The  type  of  impact  testing  machine  shown  in  Fig.  108 
is  used  for  testing  large  specimens,  especially  of  timber. 

The  significance  of  the  results  of  an  impact  test  is  a 
matter  of  some  uncertainty.  An  impact  test  of  an  un- 
notched  specimens  gives  results  which  seem  to  be  an  index 
of  the  toughness  of  the  mateiial;  the  results  of  a  test  of  a 
notched  specimen  probably  also  indicate  toughness.  The 
impact  test  has  been  claimed  to  give  an  index  of  the  resist- 
ance of  material  to  progressive  breakdown  under  repeated 
stress.  Localized  stress  arising  from  defects  in  the  mater- 
ial seems  to  affect  both  resistance  to  impact  and  resistance 
to  repeated  stress  moie  than  they  do  resistance  to  static 
stress.  The  value  of  the  impact  test  as  an  index  of  resist- 
ance to  repeated  stress  has  not  been  proved  as  yet. 

Repeated  Stress  Tests  and  Testing  Machines. — Tests  of 
the  strength  of  materials  under  repeated  stress  and  testing 
machines  for  making  such  tests  have  not  been  standard- 
ized as  have  "static"  tests  of  materials  and  "static"  test- 
ing machines,  such  as  are  described  on  pages  267-273. 
Two  types  of  testing  machine  for  repeated  stress  tests  are 
in  fairly  common  use,  and  they  are  shown  in  diagram  in 
Fig.  110. 

In  Fig.  llOa  is  shown  a  machine  which  applies  repeated 
flexural  stress  by  means  of  a  crank  and  connecting  rod, 
and  measures  the  bending  moment  applied  to  the  specimen 
by  means  of  the  compression  of  calibrated  springs.  Power 


TESTING  AND  INSPECTION:  TESTING  MACHINES  285 

is  furnished  by  a  motor  M  (or  from  a  line  shaft)  and  a  crank 
C  with  adjustable  throw  is  driven  by  the  motor.  The  crank 
is  attached  to  a  connecting  rod  R  wrhich  bends  a  specimen 
S  back  and  forth.  The  motion  of  the  specimen  is  resisted 
by  springs  G  acting  through  a  bent  lever  A .  The  amount  of 
bending  moment  applied  to  the  specimen  may  be  varied 
by  changing  the  throw  of  the  crank  and  is  measured  by  the 
amount  of  compression  of  the  springs  G.  This  compression 
is  indicated  by  the  throw  of  the  arm  7  to  the  end  of  which 
is  attached  a  pencil  which  records  the  throw  on  paper 
wrapped  round  the  drum  D.  The  drum  D  is  rotated  by 


B        B' 


(a)  (6) 

FIG.   110. — Types  of  testing  machine  for  making  repeated  stress  tests. 

a  worm  and  wheel  drive  from  the  main  shaft  of  the  machine, 
and  there  is  consequently  recorded  on  the  paper  a  diagram 
whose  width  is  a  measure  of  the  bending  moment  on  the 
specimen  and  whose  length  is  a  measure  of  the  number  of 
applications  of  stress  to  the  specimen.  The  number  of 
applications  of  stress  is  also  indicated  by  a  counter  K. 
From  the  bending  moment  the  maximum  unit  stress  ap- 
plied to  the  specimen  can  be  determined.  Usually  this 
type  of  machine  is  used  to  produce  reversals  of  bending 
stress,  but  by  varying  the  springs  G  other  stress  ranges  can 
be  applied  to  the  specimen.  The  Upton-Lewis  machine  is 
the  commonest  example  of  this  type  of  machine  used  in  the 
United  States. 

Fig.    1106  shows  in  diagram  a  testing  machine  which 


286  MATERIALS  OF  ENGINEERING 

produces  reversal  of  bending  stress  by  the  use  of  a  rotating 
flexure  specimen.  The  specimen  S  is  supported  on  ball 
bearings  B  and  driven  by  a  pulley  P.  Weights  are  hung 
from  a  second  set  of  ball  bearings  B'y  and  these  weights  set 
up  bending  stresses  in  the  specimen.  The  bending  stress 
set  up  in  the  upper  fibers  of  the  specimen  is  compression, 
and  in  the  lower  fibers,  tension.  As  the  specimen  is  ro- 
tated the  stress  for  any  fiber  changes  from  compression  to 
tension,  and  the  stress  is  completely  reversed.  The  maxi- 
mum unit  stress  for  both  tension  and  compression  can  be 
computed  from  the  amount  of  weights  W  applied,  the  dimen- 
sions of  the  specimen,  and  the  distances  between  bearings. 
As  shown  the  bending  moment  and  maximum  unit  stress 
is  uniform  between  the  two  center  bearings.  A  counter  K 
indicates  the  number  of  reversals  of  stress  given  to  the 
specimen,  and  when  the  specimen  breaks  the  counter 
automatically  stops. 

Hardness  Testing  Apparatus. — The  hardness  of  a  material 
is  usually  defined  as  its  resistance  to  plastic  deforma- 
tion. Various  forms  of  test  have  been  proposed  to  deter- 
mine hardness.  For  brittle  materials  some  form  of  scratch 
test  has  been  used.  A  diamond  point  of  some  definite 
shape  is  pressed  against  the  surface  of  the  specimen  by  a 
known  weight,  and  the  width  of  scratch  made  as  the  dia- 
mond point  is  drawn  along  is  measured  by  means  of  a  micro- 
scope. This  test  gives  rather  variable  results. 

A  very  common  type  of  hardness  test  is  made  by  forcing 
a  hardened  steel  point  of  definite  shape  into  the  surface  of 
a  sample  of  the  material  to  be  tested.  A  standard  pres- 
sure is  used  and  the  dimensions  of  the  indentation  measured. 
The  Brinell  test  is  -the  commonest  of  these  indentation 
tests.  In  the  Brinell  test  a  steel  ball  10  mm,  in  diameter 
is  forced  into  the  material  to  be  tested;  for  steel  the  stand- 
ard pressure  is  3000  kg.,  for  softer  metals  500  kg.  The 
result  of  the  test  is  given  by  a  " hardness  number,"  which 
is  the  quotient  obtained  by  dividing  the  load  by  the  area 
of  the  spherical  impression  made.  This  area  may  be 
determined  either  from  the  diameter  or  the  depth  of  the 


TESTING  AND  INSPECTION:  TESTING  MACHINES  287 

spherical  impression  left  in  the  specimen  after  the  load  is 
removed.  The  Brinell  test  can  be  made  on  a  testing 
machine  like  that  shown  in  Fig.  100  or,  as  is  the  more  com- 
mon case,  a  special  testing  machine  may  be  used. 

Another  apparatus  for  testing  hardness  which  is  in  com- 
mon use  is  the  scleroscope.  In  this  instrument  a  small 
weight  fitted  with  a  diamond  point  falls  from  a  standard 
height  to  the  surface  of  the  material  under  test.  The  height 
of  rebound,  read  from  a  scale  on  the  tube  down  which  the 
weight  falls,  gives  an  arbitrary  "  hardness  number "  for 
the  material.  The  Brinell  hardness  number  and  the 
scleroscope  hardness  number  are  not  the  same. 

Both  scleroscope  tests  and  Brinell  tests  usually  give 
a  rough  index  of  the  ultimate  tensile  strength  of  the  material 
tested,  though  the  quantitative  relation  between  hardness 
number  and  tensile  strength  varies  for  different  mater- 
ials. Both  scleroscope  tests  and  Brinell  tests  may  be 
made  without  seriously  injuring  the  material  tested.  It 
is  thus  possible  to  test  the  very  piece  to  be  used  for  a 
machine  part  or  structural  member  instead  of  being  com- 
pelled to  rely  wholly  on  tests  of  samples  which  are  sup- 
posed to  be  representative  of  the  material.  Scleroscope 
and  Brinell  tests  are  especially  useful  in  determining  the 
uniformity  of  hardness  throughout  a  machine  part. 

Other  hardness  tests  have  been  proposed  involving  the 
cutting  or  the  filing  of  a  specimen  of  the  material  tested. 
Such  tests  have  given  rather  unreliable  results  on  account  of 
the  influence  of  even  small  variations  in  the  shape  and  hard- 
ness of  the  cutting  tool  used. 

Cold-bend  Tests. — A  common  shop  test  for  ductility 
in  a  metal  is  the  cold-bend  test,  which  is  made  by  bending 
a  sample  strip  of  the  metal  to  be  tested  round  a  mandrel 
of  given  diameter,  and  noting  the  angle  of  bending  when  the 
outer  surface  of  the  metal  first  shows  a  crack.  This  test 
is  usually  made  in  a  shop  by  hammering  the  sample  round 
the  mandrel,  or  by  bending  in  a  vise  or  a  hydraulic  press. 
Special  testing  machines  have  been  devised  for  making 
cold-bend  tests  quickly  and  easily. 


28$  MATERIALS  OF  ENGINEERING 

Magnetic  Tests  of  Steel  as  an  Index  of  Mechanical 
Properties. — There  seems  to  be  a  fairly  definite  relation 
between  the  magnetic  properties  of  steel  and  the  strength 
properties.  This  relation  has  not  been  developed  so  fully 
as  to  make  standardized  magnetic  tests  for  strength  prop- 
erties feasible,  but  there  is  a  possibility  that  such  tests 
may  become  possible  in  the  near  future.  A  magnetic 
test  could  be  applied  to  actual  iron  and  steel  parts  of 
machines  without  injuring  them. 

A  study  of  the  uniformity  of  magnetic  properties  through 
out  a  bar  of  steel  gives  promise  of  being  a  useful  test  for  the 
purpose  of  discovering  flaws  and  other  defects,  and  can  be 
easily  carried  out  on  pieces  of  uniform  cross-section.  This 
test  has  already  been  applied  to  steel  rails  and  to  gun  barrels. 

There  seems  also  to  be  a  possibility  of  detecting  internal 
flaws  in  metals  by  the  use  of  very  powerful  X-ray  tubes. 

Selected  References  for  Further  Study 

UNWIN:  "The  Testing  of  the  Materials  of  Construction,"  London,  1910. 

An  excellent  treatise  by  an  eminent  British  testing  engineer. 
MARTENS  (translation  by  HENNING):  "Handbook  of  Testing  Materials," 

New  York,  1899.     Somewhat  out  of  date  but  excellent  in  its  discussion 

of  fundamental  principles.     By  the  late  director  of  the  Prussian  Royal 

Materials  Testing  Laboratory. 
"Standards"  of  the  American  Society  for  Testing  Materials,"  Published 

triennially  by  the  society   at   Philadelphia    (last  issue,     1918).     The 

volume  has  articles  on  standard  methods  of  testing. 


CHAPTER  XXI 
SPECIFICATIONS  FOR  MATERIALS 

General  Characteristics  of  Specifications  for  Materials.— 

The  codified  statement  of  the  requirements  for  accepta- 
bility of  a  mateiral  of  construction  makes  up  a  set  of  speci- 
fications for  that  material. 

Specifications  for  materials  are  formulated  by  individual 
consumers,  by  associations  of  consumers,  by  joint  confer- 
ence between  producers  and  consumers,  and  by  technical 
societies.  The  standard  specifications  for  materials  adopt- 
ed by  the  American  Society  for  Testing  Materials  furnish 
an  excellent  illustration  of  specifications  systematically 
and  carefully  drawn  up.  Various  committees  of  this  so- 
ciety draw  up  specifications  for  different  kinds  of  materials 
—thus  there  is  one  committee  for  specifications  for  steel, 
one  for  cast  iron,  one  for  cement,  one  for  road  materials, 
and  so  on.  On  each  of  these  committees  are  representa- 
tives of  the  manufacturers  of  the  material,  representatives 
of  the  consumers  of  the  material,  and,  in  some  cases,  repre- 
sentatives from  independent  testing  laboratories.  The 
specifications  proposed  by  these  various  committees  are 
confirmed  or  rejected  by  vote  of  the  society.  The  accepted 
specifications  are  then  published  triennially  in  the  "  Stand- 
ards" of  the  society.  The  specifications  of  this  society 
are  very  widely  adopted  as  standard  by  users  and  manu- 
facturers of  materials. 

The  content  of  a  set  of  specifications  may  be  divided 
into  four  subdivisions:  (1)  specifications  relating  to  meth- 
ods of  manufacture  to  be  used;  (2)  specifications  relating 
to  finish,  form  and  dimensions  of  the  pieces  of  material  in 
a  shipment;  (3)  specifications  for  the  chemical  and  phy- 
sical properties  of  the  material;  and  (4)  specifications^for 
the  methods  of  testing  to  be  used  in  determining  the  chemi- 

19  289 


290  MATERIALS  OF  ENGINEERING 

cal  and  physical  properties.  Illustrations  of  the  first 
subdivision  are  the  specifications  that  structural  steel  for 
bridges  shall  be  made  by  the  open-hearth  process,  and  that 
staybolt  iron  shall  be  made  by  the  puddling  process.  The 
second  subdivision  is  illustrated  by  the  specifications  for 
allowable  variation  between  nominal  and  actual  dimensions 
for  dressed  timber,  by  the  specifications  for  classifying 
timber  according  to  its  straightness  of  grain  and  freedom 
from  knots,  and  by  the  allowable  variations  from  nominal 
thickness  for  steel  plates.  The  third  subdivision  is  illus- 
trated by  the  requirements  for  maximum  allowable  sul- 
phur content,  and  for  transverse  strength  of  gray  cast 
iron,  by  the  maximum  allowable  phosphorus  content  and 
the  strength  requirements  for  steel,  and  by  the  maximum 
allowable  content  of  sulphur  trioxide  and  the  strength 
requirements  for  Portland  cement.  The  fourth  subdivi- 
sion is  illustrated  by  the  specifications  for  Portland  cement 
in  which  the  methods  of  testing  to  be  followed  are  set  forth 
in  great  detail. 

Specifications  are  never  perfect.  They  are  not  drawn 
up  for  ideal  material,  but  for  material  which  it  is  possible 
to  obtain  at  a  reasonable  cost  under  existing  conditions 
of  manufacture.  From  time  to  time  it  becomes  necessary 
to  change  details  of  any  standard  set  of  specifications,  and, 
in  general,  the  requirements  tend  to  become  more  and  more 
exacting  as  the  methods  of  commercial  production  are 
improved. 

Summary  of  Tests  Required  for  Materials. — A  general 
statement  of  the  content  of  current  standard  specifications 
for  materials  of  construction  can  not  be  made  in  this  chap- 
ter on  account  of  the  great  space  which  would  be  required 
even  for  a  summary.  Table  18  gives  a  summarized 
statement  of  the  kinds  of  tests  which  are  required  for  the 
principal  materials  of  engineering  for  which  standard 
specifications  have  been  adopted.  The  table  is  based  on 
the  " Standards"  of  the  American  Society  for  Testing 
Materials,  which  is  a  volume  issued  every  three  years. 
Probably  the  specifications  in  this  volume  cover  the  widest 


SPECIFICATIONS  FOR  MATERIALS 


291 


TABLE  18. — SUMMARY  OF  THE  TESTS  REQUIRED  FOR  THE  PRINCIPAL  MATE 

RIALS  OF  ENGINEERING  BY  THE  1918  "STANDARDS"  OF  THE  AMERICAN 

SOCIETY  FOR  TESTING  MATERIALS 


Material 

Process  of 
manufacture 

Chemical1 

tests 

Physical 
tests 

Special 
tests 

Steel  and  Iron 

1.  Steel  for  rails.  . 

Bess,     or     open 

For      C,       Mn, 

Drop  (impact) 

hearth 

max.    P,    max 

Si 

2-  Steel     for     ry.. 

Any     approved 

For  max.  P 

Tensile 

splice  bars 

process 

Cold-bend 

3.  Steel  for  track 

Open  hearth 

For  C  and  max. 

Tensile 

bolts 

Electric 

P 

Cold-bend 

4.  Steel  for  track 

Bess,     or     open 

Tensile 

spikes 

hearth 

Cold-bend 

5.  Structural  steel 

Open-hearth 

For  max.  P  and 

Tensile 

Tensile   testa   ef 

for    bridges, 

(Bess,     allowed 

max.  S 

Cold-bend 

full-size       eye- 

buildings, 

for  buildings) 

bars 

locomotives 

cars  and  ships 

6-  Structural 

Open  hearth 

For     max.      C, 

Tensile 

Tensile   tosts    of 

nickel  steel 

max.  Mn,  max. 

Cold-bend 

full-size       eye- 

P,  max.  S,  min. 

bars 

Ni 

7.  Rivet  steel  

Open  hearth 

For      Mn      (for 

Tensile 

Quench-bend.* 

boiler    rivets), 

Cold-bend 

Special     flatten- 

max.   P,    max. 

ing  tests 

S 

8    Carbon-steel  for 

Open  hearth 

For      C,      Mn, 

springs 

Crucible 

max.   P,   max. 

Electric 

S 

9.  Alloy  steel  for 

Open  hearth 

For      C,      Mn, 

springs 

Crucible 

max.    P,    max. 

Electric 

S,  Cr,  V,  Si 

10.    Reinforcing 

Bess.,     open 

For  max.  P 

Tensile 

bars   for   con- 

hearth, or  from 

Cold-bend 

crete 

re-rolled  rails 

11.   Steel   for   forg- 

Any     approved 

For      C,      Mn, 

Tensile 

Extensometer 

ings 

process 

max.    P,   max. 

used    to   deter- 

S,  Ni,  Cr,  V 

m  i  n  e    elastic 

limit 

12.  Steel  for  axles 

Any     approved 

For      C,      Mn, 

Tensile 

Extensometer 

and  shafts 

process 

max.    P,    max. 

Drop  (impact) 

used    to   deter- 

S 

Cold-bend 

m  i  n  e    elastic 

limit 

13.   Steel  wheels  for 

Open  hearth 

For  C,  Mn,  Si, 

Careful       meas- 

cars 

max.  P,  max.  S 

urement  of  size 

14.  Steeltires  

Open  hearth 

For  C,  Mn,  Si, 

Tensile 

Careful       meas- 

max. P,  max.  S 

urement  of  size 

15.  Steel  castings... 

Any     approved 

For     max.      C, 

Tensile 

Hammer  test  for 

process 

max.    P,    max. 

Cold-bend 

soundness     o  f 

(Castings    for 

S 

castings     for 

ships  must  be 

ships 

annealed) 

292 


MATERIALS  OF  ENGINEERING 


TABLE   18.  —  Continued 


Material 

Process  of 
manufacture 

Chemical 
tests 

Physical 
tests 

Special 
tests 

16.   Steel    boiler 

Open  hearth 

For      C,       Mn, 

Cold-flanging 

Hydraulic  press- 

tubes 

max.    P,    max. 

Flattening 

ure  test 

17.   Welded  steel 

Bess,    or     other 

Tensile 

Hydraulic  press- 

pipe 

approved    pro- 

Flattening 

ure  test 

cess 

18.  Steels  for  auto- 

Any    approved 

For      C,      Mn, 

Tensile 

Extensometer 

mobiles 

process 

max.    P,    max. 

Cold-bend 

used    to    deter- 

S,   Ni,   Cr,   Si, 

m  i  n  e    elastic 

V 

limit 

19.  Boiler  and  fire- 

Open hearth 

For      C,       Mn, 

Tensile 

Nick  bend  test* 

box  steel 

max.    P,    max. 

Cold-bend 

to  show  struc- 

S 

ture  of  steel 

20.  C  o  1  d-drawn 

Bess.,    or    open 

For      C,       Mn, 

steel  for  screw 

hearth 

max.    P,    max. 

stock 

S 

21.   Charcoal-iron 

Knobbling 

Quench-bend2 

boiler  tubes 

Nick-bends 

Hydraulic  press- 

ure 

22.  Wrought-i  r  o  n 

Puddling 

Tensile 

Hydraulic  press- 

pipes 

Cold-bend 

ure 

23.  Staybolt  iron.  .  . 

Puddling 

For  max.  Mn 

Tensile 

Etch      test4      to 

Cold-bend 

show  structure 

24.  Wrought  iron 

Puddling          j 

For  •  max.     Mn 

Tensile 

Hot-bend8  test 

for  bolts,  bars, 

(forgings  only) 

Cold-bend 

Nick-bend' 

plates,     and 

Etch  test* 

forgings 

25.   Cast-iron  pipe.  . 

Flexure 

Hydraulic  press- 

ure 

26.   Cast-iron      for 

One    yielding 

For      max.      P, 

Flexure 

Chill  test« 

locomotive 

gray  iron 

max.  S 

cylinders 

27    Cast-iron      car 

One    yielding 

Drop     (impact) 

Chill  test8 

wheels 

gray  iron 

test 

Thermal  test7 

28.  Gray-iron  cast- 

Cupola    or     air 

For  max.  S 

Flexure 

ings. 

Tensile 

29.  Malleable  cast- 

Air  furnace 

Tensile 

iron 

Open  hearth  or 

Flexure 

electric  furnace 

Non-ferrous  Metals 

30    Copper  wire 

Tensile 

Electrical      con- 

ductivity 

31    Spelter  (zinc) 

For     max.     Cd, 

max.  Fe,  max. 

Pb 

31.     Manganese 

For  Cu,  Zn,  Al, 

Tensile 

bronze 

max.  Pb 

32.  Gun  metal,  ad- 

For Cu,  Sn,  Zn 

Tensile 

miralty 

bronze,      gov- 

' 

ernment 

bronze 

33.  Bronze    trollev 

Tensile 

Electrical      con- 

wire 

ductivity 

SPECIFICATIONS  FOR  MATERIALS 

TABLE   18. — Continued 


293 


Material 

Process  of 
manufacture 

Chemical 

tests 

Physical 

tests 

Special 
tests 

34    Copper     plates 

For  Cu 

Tensile 

Hnt  hpnrll 

and  bars 
35.  Seamless     cop- 

For Cu 

Cold-beod 

per  tubing 
36.  Seamless   brass 
tubing 

37.  Brass  rod  

For    Cu,    max. 
Pb,    max.    Fe, 
max.  total  im- 
purities 
For    Cu,     max 

flanging  tests 
Flattening    and 
flanging  tests 

ure 
Hydraluic  press- 
ure 

Non-metallic 
materials 
38.  Portland 
cement 

39.  Natural  cement 

One      involving 
calcining    mix- 
ture   of    clay- 
bearing    and 
1  i  m  e-bearing 
materials  to  in- 
cipient fusion 
One      involving 

Pb,    max.    Fe, 
max.  total  im- 
purities 

For    max.    sul- 
phuric     anhy- 
dride,     and 
max.   magne- 
sium 

cold-bend  tests 

Special 
Tensile* 

Special 

Soundness* 
Time  of  set1" 
Fineness     (sieve 
test)11 

Soundness* 

40.  Drain  tile  

calcining  clay- 
bearing     lime- 
stone   at    low 
temperature 
Burnt    clay    or 

Tensile* 
Special 

Time  of  set1" 
Fineness     (sieve 
test)" 

41.  Paving  brick.  .  . 

Portland     c  e- 
ment  concrete 

Transverse11 

Freezing  and 
thawing1* 
Rattler  test   for 

42.  Yellow  pine 

abrasion1* 
Conformity      to 

timber 
43.   Rubber-lined 

Purity    of    rub- 

Tensile 

nominal      size. 
For  knots  and 
other      defects. 
For  proportion 
of  heartwood 

hose 

ber,     max.     S 
and  other  im- 
purities 

ure 

Adhesion  be- 
tween rubber 
and  fabric 
covering 

1.  The  following  symbols  are  used  for  chemical  elements:  C,  carbon;  Mn,  manganese; 
P,  phosphorus;  Si,  silicon;  S,  sulphur;  Ni,  nickel;  Cr,  chromium;  V,  vanadium,  Cd,  cadmium; 
Fe,  iron;  Pb,  lead;  Cu,  copper;  Zn,   zinc;  Al,  aluminum:  Sn,  tin;  max.  before  the  symbol 
for  an  element  indicates  that  not  more  than  a  certain  percentage  of  that  element  is  allow- 
able; min.  before  the  symbol  for  an  element  indicates  that  a  certain  minimum  percentage 
of  that  element  is  required. 

2.  Quench  bend  tests  are  made  by  heating  a  smple  red-hot,  plunging  in  water  and  then 
making  a  bend-test  on  the  suddenly  cooled  specimen. 

3.  Nick-bend  tests  are  made  by  nicking  a  specimen  and  bending  it  so  that  it  fractures 
at  the  nick,  revealing  the  texture  of  the  material. 

4.  An  etch  test  is  made  by  polishing  the  surface  of  a  specimen  and  etching  with  acid  to 
bring  out  the  structure,  and  show  slag  present. 


294  MATERIALS  OF  ENGINEERING 

field  of  any  of  the  sets  of  specifications  issued.  Other 
societies  issuing  important  sets  of  specifications  for  mater- 
ials are:  American  Railway  Engineering  Association, 
Society  of  Automotive  Engineers,  Master  Car  Builders 
Association,  American  Foundrymens  Association,  National 
Board  of  Fire  Underwriters,  Yellow  Pine  Manufacturers 
Association,  American  Concrete  Institute,  U.S.  Govern- 
ment Departments,  especially  the  War  Department  and 
Navy  Department. 

Selected  References  for  Further  Study 

"Standards"  of  the  American  Society  for  Testing  Materials,  published 
biennially  during  the  even-numbered  years.  Contains  a  large  number 
of  standard  specifications  for  materials,  and  for  standard  methods  of 
testing. 

DUDLEY:  The  Enforcement  of  Specifications,  Proceedings  of  the  American 
Society  for  Testing  Materials,  Vol.  VII,  p.  19  (1907). 

MEAD:  "Contracts,  Specifications,  and  Engineering  Relations,"  New  York, 
1916,  especially  Chaps.  XVI  and  XVII  and  Appendix  D,  a  bibliography 
of  specifications. 

5.  A  hot-bend  test  is  made  by  bending  a  flat  specimen  double  on>  itself  while  at  a  red 
heat. 

6.  A  chill  test  is  made  by  casting  a  sample  of  iron  in  a  metal  mold,  breaking  the  sample 
after  cooling,  and  examining  the  fracture.     The  usual  requirement  is  that  the  sample  shall 
show  a  certain  depth  of  chill  from  the  surface. 

7.  A  thermal  test  is  made  by  pouring  round  a  cold  wheel  a  'ring  of  molten  iron.     The 
wheel  under  test  must  withstand  the  stresses  set  up  by  the  heat  without  breaking  or  without 
serious  cracking. 

8.  The  tensile  test  for  Portland  cement  and  for  natural  cement  is  made  on  a  specially 
shaped  briquette  molded  from  cement  and  water,  or,  more  commonly  from  cement,  sand, 
and  water.     All  the  details  of  mixing  ingredients,  molding  briquettes,  storing  briquettes,  and 
testing  briquettes  are  definitely  specified  in  the  A.  S.  T.  M.  "Standards." 

9.  Soundness  tests  are  made  by  exposing  pats  of  cement  to  air,  to  water  and  to  steam. 
To  pass  the  test  the  pats  must  not  crack  or  disintegrate. 

10.  The  time  of  set  is  determined  by  noting  the  time  which  elapses  before  a  needle  of 
definite  diameter  loaded  with  a  definite  weight  falls  to  penetrate  a  small  block  of  cement 
more  than  a  certain  distance. 

11.  The  fineness  test  is  made  by  noting  the  percentage  of  material  which  passes  through 
sieves  of  various  fineness  of  mesh. 

12.  The  test  of  a  drain  tile  is  made  by  applying  a  load  along  an  element  on  the  top  of  a 
tile  laid  with  its  axis  horizontal,  the  tile  being  supported  along  an  element  at  the  bottom. 
Or  instead  of  loading  along  a  line  the  load  may  be  applied  through  a  sand  cushion,  and  the 
tile  supported  by  another  sand  cushion.     In  either  event  the  tile  is  broken  by  flexure  of 
the  sides. 

13.  Absorption  tests  are  made  by  noting  the  percentage  of  water  which  a  dry  title  will 
absorb. 

14.  Freezing   and  thawing  tests   are    made  by  subjecting  the  tile  to  alternate  freezing 
thawing  and  watching  for  signs  of  disintegration. 

15.  A  rattler  test  is  made  by  placing  bricks  and  cast  iron  spheres  inside  a  cast  iron  "bar- 
rel"   and   giving   the  barrel  a  definite  number  of  revolutions.     The  loss  of  weight  of  the 
bricks  is  a  measure  of  their  abrasion  under  wear. 


QUESTIONS 

Chapter  I 

1.  Give  illustrations  of  parts  of  structures  or  machines  for  which  strength 
is  the  prime  requisite  for  the  material  used. 

2.  Give  illustrations  of  parts  for  which  other  properties  than  strength  are 
of  prime  importance. 

3.  Name  the  kind  of  stress  set  up  in:  (a)  bolts  holding  a  cylinder  head 
in  place,  (6)  the  shell  of  a  boiler  under  pressure,  (c)  the  eyebars  in  a  bridge 
truss,  (d)  the  connecting  rod  of  a  single-acting  gas  engine,  (e)  the  connecting 
rod  of  a  steam  engine. 

4.  Give  illustrations  of  parts  of  machines  or  structures  in  which  stiffness 
is  desirable;  in  which  stiffness  is  not  desirable. 

6.  Give  illustrations  of  parts  of  machines  or  structures  which  may  be 
made  of  brittle  material ;  which  may  not  be  made  of  brittle  material. 

Chapter  II 

1.  Define  strain,  stress,  unit  strain,  unit  stress. 

2.  In  what  units  is  each  measured? 

3.  A  tension  specimen  0.500  in.  in  diameter  is  placed  in  a  testing  machine 
and  load  applied  with  the  following  results. 

Load,  Ib.  Stretch  in'S  in.,  in. 

930  0.0012 

1,860  0.0025 

3,720  0.0050 

5,580  0.0075 

6,500  0.0088 

6,980  0.0096 

7,400  0.0110 

8,200  0.0200 

11,600  Maximum 

Determine  the  proportional  limit,  the  yield  point,  the  ultimate. 

4.  Was  the  elastic  limit  determined  in  the  above  test?     About  what  would 
be  its  value? 

5.  What  is  the  theoretical  stress  at  the  point  of  contact  of  one  chain  link 
with  another? 

6.  Give  illustrations  of  parts  of  structures  or  machines  which  are  likely 
to  be  subjected  to  accidental  overload. 

7.  Give  illustrations  of  materials  for  which  the  yield  point  is  the  practical 
ultimate  strength  when  used  in  machines  or  structures. 

8.  Give  illustrations  of  materials  in  which  the  ultimate  as  determined  in 
tests  is  the  index  of  strength  when  the  material  is  used  in  a  machine  or  a 
structure. 

295 


296  MATERIALS  OF  ENGINEERING 

9.  Explain  why  a  spring  in  the  draft  rigging  of  a  freight  car  lessens  the 
injurious  effect  of  the  sudden  pull  which  comes  as  the  train  is  started. 

10.  Under  shock,  bolts  which  have  their  shanks  turned  down  to  the  diam- 
eter of  the  root  of  the  threads  give  better  service  than  do  bolts  with  shanks 
the  diameter  of  the  outside  of  the  thread;  explain. 

11.  Explain  the  increased  strength  given  to  steel  by  cold-rolling. 

12.  Steam-turbine  rotors  sometimes  give  trouble  by  becoming  loose  on 
their  shafts  due  to  the  elastic  expansion  caused  by  centrigufal  force.     Would 
this  trouble  be  diminished  by  making  the  rotors  of  a  stronger  grade  of  steel? 
Explain. 

Chapter  III 

1.  Why  is  a  sharp  corner  more  dangerous  in  a  beam  subjected  to  repeated 
stress  than  in  one  subjected  to  steady  load? 

2.  Define  cycle  of  stress,  mechanical  hysteresis. 

3.  State  the  "crystallization"  theory  of  failure  under  repeated   stress; 
the  "micro-flaw"  theory.     Which  is  generally  accepted  today?     On  what 
evidence? 

4.  What  is  the  relation  of  the  elastic  limit  or  the  proportional  limit  de- 
termined by  static  tests  to  the  ability  of  a  material  to  withstand  repeated 
stress? 

5.  Describe  briefly  a  machine  for  making  repeated  stress  tests. 

6.  What  is  the  endurance  limit  for  a  material?     How  determined? 

7.  State  the  exponential  equation  for  repeated  stress. 

8.  If  this  exponential  equation  holds  what  would  be  the  endurance  limit 
for  an  infinite  number  of  repetitions  of  stress? 

9.  The  eyebars  and  chords  of  a  railroad  bridge  are  to  be  made  of  struc- 
tural steel,  and  should  be  designed  to  withstand  2,000,000  repetitions  of 
load.     The  load  varies  from  dead  load  to  live  load  plus  dead  load,  and  the 
live  load  is  four  times  as  great  as  the  dead  load  and  sets  up  stress  in  the 
same  direction.     What  unit  stress  will  be  liable  to  cause  failure:  (a)  Con- 
sidering static   strength,    (6)   computing  the  endurance  limit    (Johnson's 
formula),  and  (c)  using  the  exponential  formula?     Is  there  greater  danger 
of  static  failure  or  of  failure  under  repeated  stress? 

10.  A  line  shaft  is  to  be  designed  to  withstand  500,000,000  reversals  of 
bending  stress,  and  is  to  be  made  of  cold-rolled  steel.     Allowing  a  "factor 
of  safety"  of  2.5  what  stress  in  bending  will  be  allowable:  (a)  Considering 
static  strength,  (b)  computing  the  endurance  limit,  (c)  using  the  exponential 
formula?     Does  static  strength  or  fatigue  strength  govern  in  this  case? 

11.  According  to  the  exponential  equation  if  the  stress  in  a  member  was 
reduced  to  one-third  its  original  value,  how  many  times  would  the  "life  "of 
the  member  be  increased? 

Chapter  IV 

1.  Define  working  stress,  factor  of  safety. 

2.  Why  must  the  working  stress  be  less  than  the  ultimate  of  the  material? 

3.  Should  higher  unit  stresses  be  allowed  in  a  wire  rope  for  a  mine  hoist 
or  in  a  wire  rope  for  a  one-story  freight  elevator?     Why? 

4.  Give  illustrations  of  parts  of  structures  or  machines  which  are  provided 


QUESTIONS  297 

to  give  insurance  against  complete  collapse  rather  than  to  carry  stress  under 
normal  conditions. 

6.  Do  the  building  laws  of  the  city  or  town  where  you  live  fix  allowable 
stresses  for  any  materials? 

6.  How  would  you  determine  the  allowable  stress  for  steel  in  the  axles  of 
heavy  trucks? 

7.  Give  your  opinion  of  the  suitability  of  the  material  used  for  railway 
and  highway  bridges  in  the  city  or  town  where  you  live. 

8.  In  your  town  or  city  is  the  construction  of  frame  (wooden)  buildings 
justifiable?     Give  reason  for  your  answer.  • 

9.  What  different  materials  are  used  in :  (a)  a  power-driven  pump,  (6)  a 
small  gasolene  engine,  (c)  a  dump  cart?     Why  is  each  material  used? 

Chapter  V 

1.  What  are  the  principal  chemical  changes  involved  in  reducing  iron  ore 
to  pig  iron? 

2.  What  is  the  reducing  agent  used? 

3.  What  is  a  flux?     Why  is  a  flux  necessary  in  reducing  iron  ore?     What 
is  the  flux  used  in  the  process  of  reducing  iron  ore? 

4.  Sketch  in  diagram  a  blast  furnace,  giving  approximate  dimensions, 
and  names  of  principal  parts. 

5.  What  is  a  hot-blast  stove?     Why  used  in  connection  with  the  blast 
furnace? 

6.  Describe  briefly  the  regenerative  process  used  in  connection  with  the 
blast  furnace. 

7.  Trace  the  air  used  in  a  blast  furnace  from  the  blowing  engines  to  the 
final  discharged  gases. 

8.  How  much  ore,  fuel,  and  flux  is  necessary  to  produce  1  ton  of  pig  iron  ? 

9.  How  much  air  is  blown  through  the  furnace  for  every  ton  of  pig  iron 
produced? 

10.  What  is  a  casting  machine? 

11.  Tell  how  the  products  of  a  blast  furnace  other  than  pig  iron  are 
utilized. 

12.  Locate  the  principal  American  iron  ore  deposits. 

13.  Locate  the  principal  American  centers  of  production  of  pig  iron. 

Chapter  VI 

1.  Define  wrought  iron. 

2.  State  the  essential  changes  involved  in  the  process  for  producing 
wrought  iron. 

3.  Sketch  in  diagram  a  puddling  furnace,  giving  approximate  dimensions. 

4.  Describe  briefly  the  process  of  producing  wrought  iron  in  a  puddling 
furnace. 

6.  What  is  the  source  of  the  oxygen  required  for  purifying  the  pig  iron? 

6.  Why  is  the  charge  in  a  puddling  furnace  kept  basic  rather  than  acid1? 

7.  What  causes  the  "fibrous"  structure  of  wrought  iron? 

8.  What  is  a  muck  ball?     A  muck  bar?     A  merchant  bar? 

9.  What  is  charcoal  iron?     For  what  used?     Why? 

10.  Why  is  wrought  iron  sometimes  adulterated  with  scrap  steel? 

11.  By  what  tests  can  wrought  iron  be  distinguished  from  steel? 


298  MATERIALS  OF  ENGINEERING 

Chapter  VII 

1.  Distinguish  between  steel  and  wrought  iron. 

2.  What  are  the   principal  chemical  changes  which  take  place  in  the 
open-hearth  process  of  making  steel? 

3.  Sketch  an  open-hearth  furnace  in  diagram,  giving  approximate  dimen- 
sions and  names  of  parts. 

4.  Describe  briefly  the  procedure  of  the  open-hearth  steel  process. 

5.  Distinguish  between  the  acid  process  and  the  basic  process  in  respect 
to:  (a)  raw  material  supplied  to  the  furnace,  (6)  ingredients  added  during 
the  process,  (c)  chemical  changes  taking  place  during  the  process,  (d)  ingre- 
dients in  the  finished  product. 

6.  Why  are  different  linings  necessary  for  acid  and  for  basic  open-hearth 
furnaces? 

7.  How  is  the  charge  drawn  from  an  ordinary  open-hearth  furnace? 

8.  What  is  a  tilting  furnace? 

9.  What  is  a  charging  machine?     Why  used? 

10.  What  is  a  recarburizer?     Why  is  a  recarburizer  usually  used  for 
open-hearth  steel? 

11.  What  are  the  functions  of  manganese  in  the  recarburizer? 

12.  How  is  it  determined  when  to  tap  an  open-hearth  furnace? 

13.  What  fuel  is  used  for  the  open-hearth  furnace? 

14.  What  limits  the  refining  action  of  the  open-hearth  furnace? 

Chapter  VIII 

1.  What  are  the  principal  chemical  changes  involved  in  making  Bessemer 
steel? 

2.  Sketch  a  Bessemer  converter  in  diagram,  giving  approximate  dimen- 
sions, and  capacity  per  charge. 

3.  Describe  briefly  the  procedure  of  the  Bessemer  process. 

4.  What  recarburizers  are  used  in  the  Bessemer  process? 

5.  Why  is  a  recarburizer  necessary? 

6.  Why  is  the  basic  Bessemer  process  not  used  in  the  United  States? 

7.  Compare  Bessemer  and  open-hearth  steel  as  to  cost,  reliability,  and 
uniformity.     Give  reasons  for  your  statements. 

8.  What  is  the  duplex  process  of  steel-making?     Why  used? 

Chapter  IX 

1.  Describe  briefly  the  cementation  process  of  making  steel. 

2.  Which  is  the  more  expensive,  cementation  steel  or  open-hearth  steel? 
Why? 

3.  Describe  briefly  the  crucible  process  of  making  steel. 

4.  Which  is  more  expensive,  crucible  steel  or  cementation  steel?     Why? 
6.  Crucible  steel  or  Bessemer  steel?     Why? 

6.  Why  do  the  cementation  and  the  crucible  processes  of  making  steel 
give  higher  grades  of  steel  than  do  the  open-hearth  and  the  Bessemer 
processes ?J 

7.  What  is  case-hardening? 


QUESTIONS  299 

8.  Why  does  an  electric  furnace  produce  a  higher  grade  of  steel  than  an 
open-hearth  furnace? 

9.  Why  is  it  not  feasible  to  conduct  the  entire  process  of  steel-making 
in  an  electric  furnace? 

10.  Sketch  in  diagram  an  arc- type  electric  furnace,  an  induction  electric 
furnace. 

11.  Why  would  it  be  feasible  to  use  electric  furnaces  for  reducing  iron 
or  in  Norway,  when  it  is  not  feasible  to  use  this  process  in  Pennsylvania 
or  Illinois? 

Chapter  X 

1.  In  what  ways  is  a  foundry  cupola  like  a  blast  furnace?     In  what 
ways  different? 

2.  What  is  air  furnace  cast  iron? 

3.  Which  is  more  expensive,  cupola  iron  or  air  furnace  iron?     Why? 

4.  What  are  the  advantages  of  the  air  furnace  over  the  cupola? 
6.  What  is  "semi-steel?"     For  what  used? 

6.  Compare  white  cast  iron  and  gray  cast  iron  in  structure,  strength, 
brittleness,  hardness.     Which  can  be  machined? 

7.  How  can  white  cast  iron  be  produced?     How  can. gray  cast  iron  be 
produced? 

8.  What  is  malleable  cast  iron?     How  produced?     For  what  used? 

9.  Compare  its  strength  and  ductility  with  the  strength  and  ductility 
of  gray  cast  iron,  of  structural  steel. 

10.  Why  are  large  "sink  heads"  often  necessary  in  molds  for  steel  cast- 
ings? 

11.  Give  illustrations  of  machine  or  structural  parts  for  which  steel  cast- 
ings are  supplanting  cast  iron;  steel  forgings. 

Chapter  XI 

1.  What  is  an  ingot?     What  is  a  "pipe?" 

2.  What  is  the  effect  of  a  pipe  on  metal  rolled  from  the  ingot?     What 
methods  are  used  to  minimize  the  danger  of  piping? 

3.  What  is  segregation?     What  ingredients  give  most  trouble  by  segre- 
gation in  steel?     What  are  the  effects  of  segregation  in  steel?     How  is 
segregation  minimized? 

4.  What  causes  unsound  or  "honeycombed"  steel?     What  is  the  effect 
on  rolled  steel  of  unsoundness?     What  methods  are  used  to  prevent  honey- 
combing? 

6.  What  is  a  soaking  pit? 

6.  What  is  a  blooming  mill?     What  is  a  two-high  mill?     A  three-high 
mill?     What  are  the  advantages  fo  each  type  of  mill?     What  is  a  "uni- 
versal" mill? 

7.  How  does  cold-rolled  steel  differ  from  hot-rolled  steel? 

8.  For  what  is  cold-rolled  steel  used? 

9.  Why  do  cold-rolled  shafts  tend  to  "kink"  if  key  ways  are  cut  in  them? 

10.  What  is  the  effect  of  annealing  on  cold-rolled  or  cold-drawn  metal? 

11.  How  is  ordinary  steel  pipe  formed? 

12.  What  is  the  difference  between  butt- welded  and  lap- welded  pipe? 


300  MATERIALS  OF  ENGINEERING 

Chapter  XII 

1.  Illustrate  by  sketch  the  charcateristic  crystalline  structure  of  pure 
metals. 

2.  Are  the  crystals  formed  perfect? 

3.  How  does  a  solution  differ  from  a  chemical  compound?     From  a 
mechanical  mixture? 

4.  What  is  a  solid  solution? 

5.  Describe  the  formation  of  structure  which  takes  place  when  a  molten 
alloy  of  tin  and  lead  solidifies. 

6.  What  is  a  eutectic? 

7.  Describe  the  solidification  of  a  molten  iron-carbon  alloy   containing 
more  than  4.3  per  cent,  of  carbon;  of  one  containing  less  than  4.3  per  cent, 
of  carbon,  but  more  than  2  per  cent,  of  carbon;  of  one  containing  less  than 
2  per  cent,  of  carbon. 

8.  Describe  the  changes  in  structure  which  occur  in  cooling  from  solidi- 
fication to  atmospheric  temperature:  (a)  for  steel  containing  more  than  0.90 
per  cent,  carbon;  (6)  for  steel  containing  less  than  0.90  per  cent,  carbon. 

9.  What  is  a  eutectoid? 

10.  Define  ferrite,  cementite,  pearlite.    austenite,  martensite,  troostite, 
sorbite. 

11.  What  is  the  recalescence  point  of  steel,  and   at  what  temperature 
does  it  occur? 

12.  Why  is  high-carbon  steel  hardened  when  suddenly  cooled? 

13.  Why  is  not  low-carbon  steel  hardened  when  suddenly  cooled? 

14.  Which  is  harder,  oil-quenched  steel,  or  water-quenched  steel?     Whv? 

15.  Why  is  the  uniformity  of  steel  in  steel  castings  improved  by  annealing? 

16.  Why  are  welded  joints  in  steel  made  tougher  by  hammering  as  they 
cool? 

17.  How  much  would  the  strength  of  steel  be  reduced  at  a  temperature 
of  400°F.?     Of  400°C.? 

18.  Define  plastic  welding,  fusion  welding,  spot  welding. 

19.  State  three  means  used  to  produce  the  very  high  temperature  required 
for  fusion  welding. 

20.  What  process  of  welding  would  you  recommend  for  making  wrought 
iron  chain?     For  mending  a  broken  punch  frame?     For  mending  a  crack 
in  a  thin  steel  plate?     For  welding  together  a  small  steel  box?     Give  reasons 
in  each  case. 

Chapter  XIII 

1.  State  the  effect  on  the  strength,  and  on  the  ductility  of  steel  (ductility 
hot  and  ductility  cold)  of  phosphorus,  carbon,  nickel,  sulphur. 

2.  About  what  carbon  content  would  be  found  in  steel  suitable  for  the 
following  uses :  milling  cutters,  boiler  plate,  carriage  springs,  bolts,  shafting, 
razor  blades,  armor  plate? 

3.  What  is  manganese  steel?     For  what  used? 

4.  What  is  "invar"  steel?     What  is  its  approximate  composition?     For 
what  is  it  used? 

6.  Name  the  alloy  steels  used  where  special  strength  is  necessary. 
6.  Explain  the  hardness  of  "high-speed"  steels  at  high  temperatures. 


QUESTIONS  301 

7.  Why  is  the  silicon  content  low  in  basic  open-hearth  steel? 

8.  High-carbon  steel  is  brittle  and  hard,  gray  cast  iron  has  a  high  carbon 
content,  but  is  brittle  and  soft;  explain. 

9.  Why  is  a  high-phosphorus  cast  iron  suitable  for  use  in  stove  foundries? 

10.  Distinguish  between  rusting  and  corrosion  (or  pitting). 

11.  Name  three  methods  used  to  lessen  the  danger  of  corrosion  in  steel. 

12.  State  the  arguments  advanced  in  favor  of  wrought  iron  as  a  corrosion 
resisting  material. 

13.  From  your  own  observation  of  structures  (windmill  towers,  highway 
bridges,  railway  signal  towers,  etc.)  and  of  water  and  gas  pipes  which  is  the 
more  resistant  to  corrosion,  wrought  iron  or  steel? 

Chapter  XIV 

1.  Describe  briefly  the  essential  steps  in  the  production  of  copper  from 
its  ores. 

2.  What  are  the  principal  uses  of  copper? 

3.  Compare  with  the  strength   and  ductility   of  structural   steel  the 
strength  and  ductility  of:  (a)  cast  copper,  (6)  hard-drawn  copper,  (c)  cast 
aluminum,  (d)  rolled  aluminum,  (e)  cast  zinc,  (/)  rolled  zinc. 

4.  Name  the  principal  uses  of  aluminum. 

5.  Why  is  aluminum  used  for  long-span  electric  transmission  wires? 

6.  Define  brass,  bronze. 

7.  What  is  the  composition  for  maximum  strength  and  for  maximum 
ductility  of  brass?     Of  bronze? 

8.  Compare  the  maximum  strength  for  brass  and  for  bronze  with  that  of 
structural  steel. 

9.  Why  is  brass  or  bronze  preferable  to  steel  as  a  bearing  metal? 

10.  Why  preferable  to  cast  iron? 

11.  What  advantage  has  brass  or  bronze  over  Babbitt  metal  as  a  bearing 
metal? 

12.  What  are  the  advantages  of  Babbitt  metal  or  others  of  the  soft  bear- 
ing alloys? 

13.  Name  several  special  non-ferrous  alloys,  give  their  special  character- 
istics, and  compare  their  strength  and  ductility  with  that  of  structural  steel. 

14.  Why  is  cold-drawn  brass  more  likely  to  give  trouble  by  "season" 
cracking  that  hot  worked  brass? 

16.  Discuss  the  minimizing  of  the  tendency  of  brass  to  give  trouble  by 
season  cracking  by  means  of  "springing"  brass  rods. 

Chapter  XV 

1.  What  are  "hard"  woods?     "Soft"  woods? 

2.  Name  several  species  of  each. 

3.  State  the  principal  sources  of  supply  in  the  United  States  of  yellow 
pine,  of  Douglas  fir,  of  white  pine,  of  other  soft  woods,  of  oak,  of  hickory, 
of  other  hard  woods. 

4.  What  are  annual  rings? 

6.  What  is  heartwood?     Sap  wood?     Spring  wood?     Summer  wood? 
6.  Describe  briefly  the  structure  of  soft  wood;  of  hard  wood. 


302  MATERIALS  OF  ENGINEERING 

7.  Why  is  it  necessary  to  give  more  attention  to  figuring  shearing  stresses 
in  timber  beams  than  in  steel  beams? 

8.  Give  values  for  the  strength  in  compression  and  in  shear  of  yellow 
pine,  Douglas  fir,  oak,  hickory. 

9.  Why  have  wooden  railway  ties  been  found  superior  to  metal  or  to 
concrete  ties? 

10.  Would  a  higher  unit  stress  be  allowable  in  a  piece  of  white  pine  for  a 
strut  in  an  aeroplane  frame,  or  in  a  large  bridge  stringer  of  white  pine? 
Why? 

11.  A  wooden  beam  is  supported  at  each  end  and  carries  a  load  in  the 
middle,  is  a  knot  more  injurious  on  the  upper  side  or  on  the  lower  side? 
Why? 

12.  What  kinds  of  knots  are  most  injurious  to  timber? 

13.  About  how  much  will  the  strength  of  air-seasoned  hemlock  be  reduced 
if  it  absorbs  water  until  its  moisture  content  is  18  per  cent.? 

14..  What  is  the  effect  of  long-continued  dead  load  on  the  resistance  of 
wood? 

16.  If  a  working  stress  of  1,000  Ib.  per  square  inch  in  compression  is 
allowed  for  yellow  pine,  and  a  short  post  is  made  of  timber  nominally  12  in. 
by  12  in.,  what  is  the  load  it  can  carry  safely  if  made  of  sawed  lumber?  If 
made  of  lumber  dressed  on  all  four  sides? 

16.  Explain  the  cracking  arid  warping  of  timber  under  improper  seasoning. 

17.  What  is  "quarter-sawed"  wood? 

18.  What  is  a  slab? 

19.  Which  makes  the  better  lumber?     Why? 

20.  What  causes  decay  of  wood  ? 

21.  Why  does  well-seasoned  wood  decay  less  than  poorly  seasoned  wood? 

22.  How  do  chemical  wood  preservatives  prevent  decay? 

23.  What  are  the  preservatives  commonly  used  for  wood? 

24.  What  are  the  advantages  and  drawbacks  of  each? 

26.  Give  a  brief  outline  of  the  process  of  treating  wood  with  preservative. 

26.  What  is  the  effect  of  the  preservative  process  on  the  strength  of  wood? 

27.  How  much  is  the  life  of  a  wooden  railway  tie  lengthened  by  the  use  of 
preservative? 

28.  Why  are  white  oak  ties  not  creosoted? 

29.  For  what  general  classes,  of  structures  is  wood  used?     Give  examples. 

30.  For  what  classes  of  structures  is  it  unsuitable?     Give  examples. 

31.  What   is   plywood?     What   advantage  does  it  have  as  a  structural 
material  over  ordinary  timber? 

32.  Why  is  plywood  made  up  with  an  odd  number  of  plies? 

33.  Why  are  I-beams  of  plywood  practicable,  while  I-beams  of  ordinary 
timber  are  not  practicable? 

Chapter  XVI 

1.  Name  the  general  classes  of  building  stone. 

2.  What   is   riprap?     Uncoursed   rubble?     Coursed   rubble?     Squared 
stone  masonry?     Cut  stone  masonry?     Ashlar  masonry? 

3.  Give  average  values  for  strength  in  compression,  in  shear,  and  in 
cross-bending  for  granite,  limestone,  and  sandstone. , 


QUESTIONS  303 

4.  For  what  general  classes  of  structures  is  stone  used? 

5.  Described  briefly  the  general  process  of  making  brick  or  other  burnt- 
clay  structural  material. 

6.  What  is  pressed  brick?     Repressed  brick?     Face  brick?     Firebrick? 

7.  How  do  paving  bricks  differ  from  building  brick  ? 

8.  What  is  terra-cotta? 

9.  How  is  soft  terra-cotta  made?     For  what  used? 

10.  How  does  sewer  pipe  differ  from  ordinary  drain  tile? 

11.  What  is  sand-lime  brick  ? 

12.  About  what  load  would  cause  failure  in  compression  if  applied  to  a 
pier  12  in.  by  12.  built  of  common  brick  with  mortar  joints?     If  built  of 
face  brick  with  Portland-cement  mortar?    If  built  of  terra  cotta  blocks 
with  Portland-cement  mortar? 

13.  What  causes  tend  to  shorten  the  life  brick  masonry? 

14.  In  what  general  classes  of  structures  are  the  burnt  clay  products  used? 

15.  How  is  terra-cotta  used  as  a  fireproofing  material? 

Chapter  XVII 

1.  What  is  the  general  property  of  cementing  materials  which  makes 
them  useful  in  structures? 

2.  Describe  briefly  the  production  of  gypsum? 

3.  What  are  the  general  uses  of  gypsum  as  a  structural  material? 

4.  Why  are  gypsum  blocks  made  larger  than  ordinary  bricks? 
6.  Hovy  does  gypsum  act  as  a  fireproofing  material? 

6.  What  is  the  average  strength  in  compression  of  carefully  made  gyp- 
sum? 

7.  What  are  some  factors  affecting  this  strength? 

8.  Outline  the  process  of  producing  quicklime. 

9.  What  is  hydrated  lime?     For  what  used? 

10.  What  is  natural  cement?     Puzzolan  cement? 

11.  Can  lime  mortar  be  used  under  water? 

12.  Can  natural  cement  mortar  be  used  under  water? 

13.  Define  Portland  cement. 

14.  From  what  is  it  made? 

16.  Outline  the  process  of  its  manufacture. 

16.  Draw  a  diagram  showing  the  general  scheme  of  a  cement  manufac- 
turing plant. 

Chapter  XVIII 

1.  Define  aggregate,  fine  aggregate,  coarse  aggregate,  mortar. 

2.  Would  it  be  advisable  to  use  plain  concrete  or  reinforced  concrete  for: 
(1)  The  foundation  for  a  small  dwelling,  (2)  the  girders  for  a  short-span 
bridge,  (3)  a  short-span  arch,  (4)  a  long-span  arch,  (5)  concrete  building 
blocks,  (6)  concrete  floor  slabs?     State  reasons  in  each  case. 

3.  Why  is  concrete  aggregate  frequently  washed  before  being  mixed 
with  concrete? 

4.  Give  illustrations  of  concrete  construction  for  which  it  would  be 
advisable  to  use  the  simpler  methods  of  proportioning;  for  which  the  more 
elaborate  methods,  based  on  sieve  analysis,  would  be  advisable.     Give 
reasons. 


304  MATERIALS  OF  ENGINEERING 

6.  Define  voids,  well-graded  aggregate.  How  and  why  does  the  use  of 
well-graded  aggregate  tend  towards  low  cost  with  good  construction? 

6.  State  some  common  proportions  used  for  mixing  the  ingredients  of 
concrete,  and  indicate  for  what  class  of  work  each  \vould  be  used. 

7.  What  is  a  sieve  analysis  of  aggregate? 

8.  Why  is  a  batch  mixer  considered  better  than  a  continuous  mixer? 
Give  an  illustration  of  a  piece  of  concrete  construction  for  which  you  would 
advise  hand  mixing. 

9.  Discuss  the  effect  of  time  of  mixing  on  the  strength  of  concrete. 

10.  How  may  the  quality  of  concrete  be  injured  during  placing  (1)  on 
ordinary  work,  (2)  under  water? 

11.  Discuss  the  use  of  water  (1)  in  mixing  concrete,  (2)  in  curing  concrete. 

12.  What  is  the  unit  system  for  casting  concrete?     What  is  a  monolithic 
structure?     Why  is  it  allowable  to  use  leaner  mixtures  for  making  concrete 
blocks  than  for  monolithic  concrete? 

13.  Give  average  values  for  the  strength  of  concrete  in  compression;  in 
shear.     Give  an  illustration  of  a  concrete  structural  member  in  which  shear- 
ing strength  of  material  is  important. 

14.  Discuss  the  use  of  "deformed"  bars  for  concrete  reinforcement. 

16.  Compare  the  ratio  of  working  stress  to  ultimate  given  for  concrete 
with  the  ratio  allowable  for  steel ;  with  the  ratio  for  timber. 

16.  State  precautions  to  be  observed  when  laying  concrete  in  cold  weather. 

17.  What  is  the  relation  between  the  waterproof  qualities  of  concrete  and 
its  resistance  to  disintegration?     What  are  some  causes  of  the  disintegration 
of  concrete? 

18.  What  is  the  action  of  concrete  under  heat  such  as  would  be  caused  by 
a  conflagration?     Give  your  opinion  of  the  relative  advantages  as  fire- 
proofing  material  of  gypsum,  terra-cotta,  and  concrete. 

Chapter  XIX 

1.  Outline  the  production  of  rubber. 

2.  Compare  rubber  with  (a)  structural  steel,  (b)  white  oak,  (c)  portland 
cement  concrete  with  respect  to:  (1)   Strength,    (2)   stiffness,    (3)   elastic 
resilience,  (4)  work  required  for  rupture  in  tension,  (5)  mechanical  hysteresis 
for  a  stress  one-half  the  ultimate  tensile  strength. 

3.  What  property  of  rubber  would  be  a  measure  of  ability  to  give  good 
wear  in  the  tires  of  a  racing  automobile? 

4.  How  large  an  oak  block  would  be  required  to  absorb  the  same  amount 
of  energy  which  can  be  absorbed  by  a  rubber  buffer  6  inches  in  diameter 
and  2  inches  thick,  allowing  the  rubber  buffer  to  be  compressed  to  1  inch 
thick,  and  allowing  a  stress  of  3000  Ib.  per  sq.  in.  in  the  oak.     (Use  stress- 
strain  diagram  for  soft  rubber  in  compression  as  given  in  text.) 

5.  Why  is  rawhide  more  suitable  for  gears  than  tanned  leather,  while 
the  reverse  is  true  for  belting? 

6.  What  kind  of  belting  would  you  recommend  for:  (1)  driving  a  tool- 
room lathe  (2)  driving  a  threshing  machine  (3)  driving  a  dynamo  from  a 
water  wheel?     State  reasons  in  each  case. 

7.  How  large  a  Manila  rope  would  you  recommend  to  replace  a  hoisting 
chain  with  links  made  from  wrought  iron  rod  %  inch  in  diameter? 


QUESTIONS  305 

Chapter  XX 

1.  Distinguish  between  the  terms  inspection  and  testing  as  applied  to 
materials. 

2.  Discuss  briefly  the  importance  of  careful  sampling  of  materials  which 
are  to  be  tested. 

3.  Give  illustrations  of  chemical  tests  used  commercially  in  testing  mate- 
rials; of  microscopic  tests;  of  tests  of  strength;  of  tests  of  ductility;  of  tests 
of  hardness;  of  other  tests. 

4.  Describe  briefly  the  mechanism  of  a  testing  machine  for  applying 
force,  for  weighing  force,  and  for  making  tests  in  tension,  compression,  and 
cross-bending. 

6.  What  are  the  advantages  and  limitations  of  (1)  the  compound-lever 
type  of  testing  machine,  (2)  the  hydraulic  press  type  of  testing  machine, 
and  (3)  the  Emery  type  of  testing  machine? 

6.  In  what  units  are  the  results  of  an  impact  test  of  a  specimen  on  a 
pendulum-type  machine  given?     How  could  a  comparable  result  be  ob- 
tained from  the  stress-strain  diagram  of  a  specimen  obtained  on  a  "static" 
testing  machine. 

7.  What  kind  of  a  testing  machine  would  be  used  to  obtain  the  shearing 
strength  and  stiffness  of  material? 

8.  What  are  the  advantages  and  disadvantages  of  the  two  types  of 
repeated  stress  testing  machine  shown  in  the  text. 

Chapter  XXI 

1.  Why  are  chemical  tests  for  phosphorus  and  sulphur  required  for 
many  kinds  of  steel?     Why  are  they  not  required  for  wrought  iron? 

2.  Why  are  special  quench-bend  tests  required  for  rivet  steel? 

3.  Why  are  drop  tests  required  for  steel  foe  axles,  shafts,  and  rails? 

4.  What  is  the  reason  for  the  requirement  of  a  hydraulic  pressure  test 
for  iron  and  steel  pipes  and  tubes? 

5.  Why  are  flexure  tests  usually  used  for  cast  iron  rather  than  tensile 
tests? 

6.  What  is  the  significance  of  each  of  the  several  tests  required  for  Port- 
land cement? 

7.  What  elementary  stresses  are  set  up  in  the  specimen  used  for  tests 
of  drain  tile? 

8.  Why  is  the  rattler  test  used  for  paving  brick? 

9.  What  is  the  significance  of  cold-bend  tests  for  steel  and  iron?     What 
test  results  from  a  tensile  test  are  comparable  with  the  results  of  a  cold- 
bend  test? 


INDEX 


Acid  steel  and  basic  steel,  86,  98 
Aggregates  concrete,  206 
coarse,  207 

desirable  qualities  in,  206 
fine,  207 

fineness  modulus,  220 
granulimetric    composition    of 

217 

light  weight,  206 
proportioning  for  concrete,  209 
sieves  for  testing,  216 
specific  gravity,  210 
surface  area,  230 
undesirable  ingredients  in,  207 
voids  in,  209 
well-graded,  207 
Air-furnace  iron,  110 
Alloys  aluminum,  161 
non-ferrous,  156 
special,  161 
three-metal,  159 
see  also  brass  and  bronze 
Aluminum  alloys,  161 
bronze,  162 
properties  of,  155 
reduction  of,  154 
strength  of,  155 
uses  of,  154 
American      Society      for      Testing 

Materials,  289 
elastic  limit,  279 
Annealing  steel,  134,  137 
Arc-type  electric  furnace,  105 
Arc  welding,  139 
Austenite,  132 
Axial  load  and  flexure,  22 
Axis,  neutral,  14 
principal,  17 


B 


Babbitt  metal,  164 

Basic jsteel  and  acid  steel,  86,  98 


307 


Beams,  bending  moments,  15 
curved,  18 
distribution  of  shearing  stress 

25 

horizontal  shearing  stress,   24 
obliquely  loaded,  17 
shear  in,  15,  24,  174 
shearing   stress   for   composite 

sections,  26 
symmetrical  and  unsymmetrical 

sections,  17,  19 
vertical  shearing  stress,  24 
Bearing  metals,  163 
Bending  moment  in  beams,  15 
Belting,  canvas,  260 
leather,  259 
rubber,  260 

weight  and  strength  of,  259,  260 
Belt  joints,  strength  of,  259 
Bessemer  process  for  copper,  152 
Bessemer  steel,  basic,  98 
converter,  94 

general  features  of  process,  94 
operation  of  converter,  95 
pig  iron  for,  94 
recarburization,  97 
uses,  98 

vs.  crucible  steel,  104 
vs.  open-hearth  steel,  99 
Blast  furnace,  73 

chemical  changes  in,  75 

hot  stoves,  76 

proportions   of   ore,    fuel,    and 

flux,  73 

•'regenerative"  process,  77 
temperatures,  76 
utilization  of  slag,  78 
utilization  of  waste  heat,  77 
see  also  pig  iron 
Blister  steel,  101 
Blow-holes  in  steel  ingots,  117 
Brass,  157 

strength  of,  157 
cracking  of,  159 


308 


INDEX 


Brick,  classification  of,  190 

durability,  194 

firebrick,  190 

manufacture,  189 

masonry,  strength  of,  193 

paving,  190 

sand-lime,  193 

strength  of,  193 
Brinell  test  for  hardness,  287 
Brittleness,  3 
Bronze,  158 

strength  of,  158 

cracking  of,  159 


Canvas  belting,  260 
Carbon  in  steel,  143 

in  cast  iron,  111 

in  wrought  iron,  84 
Case-carbonized    or    case-hardened 

steel,  101 
Castings,  iron,  110 

steel,  113 
Cast  iron,  109 

air-furnace,  110 

chilled,  111 

crystallization   during    cooling, 
129 

cupola,  109 

graphite  in,  111,  129 

gray,  111 

malleable,  112 

open-hearth,  110 

white,  111 

Cementation  steel,  101 
Cementing  materials,  197 
Cement,  natural,  200 

Portland,  see  Portland  cement 

proportioning  for  concrete,  209 

puzzolan,  200 

slag,  200 

see  also  concrete 
Cementite,  130 
Charcoal  iron,  85 
Chilled  cast  iron,  111 
Chrome-nickel  steel,  147 
Classification  of  materials,  5 
Coefficient  of  expansion,  40,  252 


Cold-bend  tests,  287 

Cold-rolled  and  cold-drawn  metals, 

36 

steel,  119 

strength  and  ductility,  121 
Columns,  26 

fixed-ended,  27 
pin-ended,  27 

Rankine-Gordon  formula,  27 
straight  line  formula,  27 
Combined  stresses,  22 

axial  stress  and  flexure,  22 
considering  lateral  strain,  29 
tensile    (or    compressive)    and 

shearing  stress,  23 
Compression,  12 
Compression    members,     long,     see 

columns 
Compressive    stress    and    sheaiing 

stress,  23 

in  torsion  members,  21 
Compressometers,  277 
Concrete,  Portland  cement,  205 

aggregate,         see         aggregate; 

blocks,  242 

calculation  of  water  in,  224 
coefficient    of    expansion,     40, 

252 

comparison  of  methods  of  pro- 
portioning, 233 
curing  of,  239 
deformed  bars  for  reinforcing, 

248 

density  of,  209 
disintegration  of,  250 
effect  of  age  on  strength,  245 
effect   of   low   temperature    on 

strength,  241 

effect  of  storage  on  strength,  240 
effect    of    time    of    mixing    on 

strength,  237 
effect  of  water  on  strength,  220, 

247 

electrolysis  in,  241 
fireproofing  with,  251 
handling  and  placing,  237 
hand  mixing,  235 
machine  mixing,  236 
mixing,  234 


INDEX 


309 


Concrete,   mixtures,    design   of    by 
fineness  modulus,  224 

mixtures,  steps  in  the  design  of, 
225 

modulus  of  elasticity,  244 

molds  for,  242 

monolithic,  242 

plain,  205 

reinforced,  206 

removal  of  forms,  243 

strength  of,  244 

strength  in  bond,  248 

strength  in  compression,  245 

strength  in  shear,  246 

test,   colorimetiic  of  sand  for, 
208 

unit  casting,  242 

waterproofing,  250 

wear  of,  248 

working  stresses  for,  249 

see    also    proportioning,  aggre- 
gate, and  cement 
Converter,  see  Bessemer  steel 
Copper,  cold-rolled  and  cold-drawn, 
154 

electrolytic,  153 

ores,  152 

reduction  of,  152 

in  steel,  148. 

strength  and  ductility,  153 

-tin  alloys,  158 

uses  of,  153 

-zinc  alloys,  157 
Corrosion  of  iron  and  steel,  148 

wrought  iron  vs.  steel,  149 
Creosote  for  preserving  wood,  184 
Critical  temperature  of  steel,  131 
Crucible  steel,  102 

furnace,  103 

vs.   Bessemer  and  open-hearth 

steel,  104 

"Crystallization"  of  metals,  45 
Cupola,  cast  iron,  109 
Curved  beams,  18 
Cycle  of  stress,  42 

D 

Defects  in  wood,  180 
Deflectometers,  277 


Deformation,  2 
Drain  tile,  191 
Drop-forged  steel,  123 
Ductility,  3 

cold-bend  tests  for,  2$7 
Duplex  process  for  steel,  99,  105 
Durability,  4 


Elasticity,  3 

modulus  of,  38 

modulus  of  for  concrete,  244 

modulus  of,  not  a  measure   of 

strength,  39 
Elastic  limit,  31 

A.  S.  T.  M.,  279 

determination  of,  277 

effect  of  cold-working  on,  36 

Johnson's  278 

materials  stressed  beyond,  34 

relation  to  pi  oportional  limit,  33 

significance  of,  32 

useful  limit,  279 

see  also  proportional  limit 
Electric  furnace  steel,  104 

cost  of  heat,  104 

quality,  107 

uses,  107 

Electric  furnace,  types,  105 
Electric  process  for  reducing  iron  ore, 

107 

Electric  welding,  138,  139 
Endurance  limit,  48 
Engineer,  the  testing,  263 
Eutectic,  127 
Eutectoid,  130 

Expansion,  coefficient  of,  40,  252 
Extensometers,  273 


Failure    of    material,    consequences 

of,  61 

theories  of,  28 
Factor  of  safety,  62 
Fatigue  of  materials,   see  repeated 


Ferrite,  130 


310 


INDEX 


Ferromanganese,  90,  97 

Fineness  modulus  for  concrete,  219 

maximum  values  of,  226 
Finish,  effect  of  in  repeated  stress,  55 
Firebrick,  190 
Fireproofing,  concrete,  25 J 

gypsum,  198 

terra-cotta,  191 
Flaws  in  metal,  effect  in  repeated 

stress,  56 
Flexure,  14 

and  axial  load,  22 

formula,  16 

see  also  beams 
Force  of  a  blow,  37 
Forged  steel,  122 

drop-forging,  123 
Forms  for  concrete,  242 

G 

Gas  in  ingots,  118 
Grain  size  of  steel,  135 
Gray  cast  iron,  111 
Guest's  theory  of  failure,  30 
Gypsum,  196 

as  fireproofing,  198 

products,  manufacture,  196 

products,  uses,  197 

strength  of,  198 

H 

Hardness,  4 

Brinell  test  for,  287 

scleroscope  test  for,  287 

testing,  286 
Hard  wood,  165 
Heat,  expansion  under,  40  - 

effect    on   strength    of   metals, 

141 

Heat-treating  steel,  134 
Hooke's  law,  11 
Hydrated  lime,  199 
Hysteresis,  mechanical,  43,  257 


Impact,  elastic  resistance  to,  37 
resistance  of  wood  to,  176 
resistance  of  rubber  to,  257 


Impact,  resistance  to,  37 

tests,  279 

Inertia,  moment  of,  16 
Ingots,  steel,  115 

blowholes  in,  117 

defects  in,  116 

gas  in,  118 

piping  in,  116 

segregation  in,  117 
Inspection  of  materials,  263 
Invar  steel,  147 

Iron-carbon  alloys,  cooling  of,  128 
Iron,  commercial  pure,  143 

corrosion  of,  148 

crystallization  of,  125 

direct  production  of,  70,  107 

occurrence  in  nature,  70 

see  also  cast  iron  and  steel 
Iron  ores,  70 

flux  used  in  reducing,  73 

fuel  for  reducing,  72 

high-grade,  70 

low-grade,  70 

mining,  72 

preparation,  72 

reduction,  72 

in  U.  S.,  71 


K 

Knobbled  iron,  85 
Knots  in  wood,  173,  180 


Laitance  in  concrete,  239 
Leather,  259 

belting,  259 

rawhide,  259 

strength  of,  259 

tanned,  259 

weight  of,  259 
Lime,  199 

hydrated,  199 
Localized  stress,  45 
Logging,  167 

Longitudinal  shear,  19,  24,  174 
Lumber,  see  wood 


INDEX 


311 


M 

Machines,  materials  for,  65 

working  stresses,  64 
Malleability,  3 
Malleable  cast  iron,  112 
Manganese  in  steel,  90,  97,  146 
Manganese  bronze,  162 
Martensite,  132 

Masonry,     brick     and     terra-cotta, 
strength  of,  193 

brick  and  terra-cotta  durability 
of,  194 

stone,  strength  and  durability, 
188 

stone,  varieties  of,  188 
Mechanical  hysteresis,  43 
Modulus  of  elasticity,  38,  244 

fineness,  219 

of  rupture,  19 

section,  16 

section  for  torsion,  21 
Moisture  in  wood,  169,  178 
Molds  for  concrete,  242 
Molybdenum  steel,  147 
Moment  of  inertia,  16 

for  composite  sections,  17,  18 

polar,  20 
Monel  metal,  163 
Mortar,  207 

N 

Natural  cement,  200 
Neutral  axis,  14 
Nickel  steel,  146 

O 

Oblique  loads  on  beams,  17 
Open-hearth  furnace  for  steel,  87 

for  cast  iron,  110 

lining  of,  89 

tilting,  90 
Open-hearth  steel,  86 

arrangement  of  plant,  92 

basic  and  acid,  86 

charging  machine,  89 

charging  the  furnace,  89 

contiol  of  process,  89 

fuel  for  processes,  92 


Open-hearth  steel,  furnace,  87 

general  features,  86 

limitations  of  pioces&,  92,  104 

recarburization,  90 

uses  of,  92 

vs.  Bessemer,  99 

vs,  crucible  steel,  104 
Overload  on  structure  or  machine,  61 
Overstress  removed  by  annealing  137 
Oxyacetylene  welding,  138 


Paving  brick,  190 
Pearlite,  130 
Phosphor  bronze,  163 
Phosphorus  in  steel,  86,  145 

in  cast  iron,  145 

in  wrought  iron,  81 
Photomicrographs,  126 
Pig  iron,  77 

for  Bessemer  steel,  94 

forge  pig,  81 

machine  casting,  78 

"pigs,"  78 

production  of,  77 

sand  casting,  78 

see  also  blast  furnace 
Pillars,  see  columns 
Piping  in  ingots,  116 
Pipe,  rolling,  123 
Plain  concrete,  205 
Plasticity,  3 

Plastic  state,  action  of  metals  in,  34 
Plywood,  182 
Poisson's  ratio,  28 
Polai  moment  of  inertia,  20 
Porcelain,  strength  of  193 
Portland  cement,  201 

manufacture,  202 

raw  materials,  201 

weight  of,  202 

see  also  concrete 
Principal  axis,  17 
Pressed  steel,  122 
Proportional  limit,  31 

determination  oi,  278 

relation  to  elastic  limit,  33 

see  also  elastic  limit 


312 


INDEX 


Proportioning     concrete     aggregate 

and  cement,  209 
Abrams  fineness  modulus,  219 
application       of       mechanical 

analysis,  214 
by  arbitrarily  selected  volumes, 

211 

by  surface  area,  230 
by  trial  mixtures,  213 
by  voids  in  aggregate,  212 
comparison  of  methods,  233 
Fuller         and         Thompson's 

method,  218 
Puddling  furnace,  81 
Puzzolan  cement,  200 


Rankine's  theory  of  failure,  30 
Rawhide,  259 
Recalescence  of  steel,  131 
Recarburization  of  steel,  90,  97 
Regenerative  process,  77,  87 
Reinforced  concrete,  206 
Repeated  stress,  42 

importance  of,  42 

compaied  with  static  stress,  42, 
46 

"crystallization,"  45 

effect  of  flaws,  56 

effect  of  outline  of  member,  55 

effect  ol  rapidity  of  repetition, 
54 

effect  of  rest,  55 

effect  of  sharp  corners,  46,  55 

effect  of  surface  finish,  55 

endurance  limit,  48 

exponential  equation  49,  51,  53 

localized  stress,  45 

mechanical  hysteresis,  43 

probability  factor,  49 

range  of  stress,  50 

resistance  of  cold-worked  metal 
to,  37 

service  of  various  machine  and 
structural  members,  56 

slip  lines,  43 

testing  machines,  284 

tests,  47 


Repeated  stress,  working  stress,  64 

wrought  iron  vs.  steel,  53 
Rest,  raises  elastic  limit,  36,  55 

effect  in  repeated  stress,  55 
Rolled  steel,  cold-drawn  and  cold- 
rolled,  119 

rolling  mill,  118 

"skelp"  123 

uses,  115 

vs.  steel  castings,  114 

see  also  rolling  mill 
Rolling  mill,  118 

three-high,  119 

two-high,  119 

universal,  119 

Rope,  weight  and  strength  of,  260 
Rubber,  254 

belting,  260 

crude,  255 

deterioration  of,  258 

energy  absorbed  by,  257 

general  characteristics,  254 

"mechanical  hysteresis,"  257 

physical  properties,  255 

production  of,  254 

resistance  to  impact,  257 

strength  of,  255 

soft  and  hard,  255 

vulcanized,  255 


Safety,  factor  of,  62 
Sampling  materials  for  testing,  264 
Sand,  see  aggregate 
Sand-lime  brick,  193 
Scleroscope,  287 
Section  modulus,  16 
Segregation  in  steel,  116 
Semi-steel,  110 
Sewer  pipe,  191 

Siemens-Martin     steel     see     open- 
hearth  steel 

Sieve  analysis  curves,  217 
Sieves  for  testing  aggregate,  216 
Sieve  shakers,  216 
Shafting,  cold-rolled,  121 
Shear,  13 

in  beams,  15,  24 

in  torsion,  19 


INDEX 


313 


Shearing     stress     and     tensile      (or 
compressive)  stress,  23 

for  composite  section,  26 

distribution  in  beams,  25 

on  inclined  section,  14 

safe,  20 

Shear  steel,  101 
Silicon  in  steel,  144 
Slag  cement,  200 

Slag,  utilization  of  blast  furnace,  78 
Slip  lines,  43 
Soft  wood,  165 
Solid  solutions,  126 
Sorbite,  134 
Specifications  for  materials,  289 

A.  S.  T.  M.,  289 

societies  issuing,  294 
Specific    gravity    of    concrete    ag- 
gregate, 210 

Specimens  for  testing  materials,  265 
Spiegeleisen,  97 
Spot  welding,  138 
St.  Venant's  theory  of  failure,  30 
Statically  determinate  member,  16 

indeterminate  members,  16 
Static  moment  of  section,  26 
Static   stress   and    repeated    stress, 

compared,  42,  46 
Steel,  annealing,  134 

carbon  in,  143 

case-carbonized    01    case- 
hardened,  101 

castings,  113 

chromium  in,  14? 

cobalt  in,  147 

cooling  of,  128 

copper  in,  148 

corrosion  of,  148 

critical  temperature,  131 

crystallization  of,  129 

distinguished    from   wrought 
iron,  81,  84 

duplex  process,  99,  105 

effect  of  hammering,    pressing, 
and  rolling,  114,  122,  136 

effect  of  temperature,  141 

eutectoid,  130 

forging,  122 

grain  size  of,  135 


Steel,  heat-treating,  134 

hypereutectoid,  130 

hypoeutectoid,  130 

ingots,  115 

magnetic  test  of,  288 

magnetic  tests  of,  288 

manganese  in,  146 

molybdenum  in,  147 

nickel  in,  146 

phosphorus  in,  145 

recalescence,  131 

refining  of  grain,  135 

silicon  in,  144 

strength,  150 

sulphur  in,  145 

tempering,  134 

titanium  in,  148 

triplex  process,  105 

tungsten  in,  147 

vanadium  in,  147 

vs,  wrought  iron,  53,  149 

welding  see  welding 

see  also  open-hearth,  Bessemer, 
crucible,  and  electric  fur- 
nace steel 

Stiffness  of  materials,  2 
Stone,  crushed  see  aggregate 

masonry,  188 

quarrying,  188 

strength  of,  188 

uses  of,  187 

varieties  of,  187 
Stoneware,  strength  of,  193 
Stoves  for  blast  furnace,  76 
Strain,  2,  10 

lateral,  28 

measurement  of,  273 
Strength  of  materials,  1 
Stress,  10 

beyond  yield  point,  35 

cycle  of,  42 

non-uniform,  14 

elementary,  1 

combined,  22,  29 

in  flexure,  14 

repeated,  see  repeated  stress 

-strain  diagram,  30,  35,  37 

in  torsion,  19 

uniformly  distributed,  12 


314 


INDEX 


Stress,  working,  59,  62,  63 
Structures,     materials     for    various 

types,  65 

Struts,  see  columns 
Sulphur  in  steel,  90,  97,  145 
Surface    area    and    compressive 

strength  of  concrete,  230 
proportioning  concrete  by,  230 


Temperature,  effect  on  strength,  141 

Tempering  steel,  134 

Tensile  stress  and  shearing  stress,  23 

in  torsion  members,  21 
Tension,  12 
Terra-cotta,  191 

blocks,  191 

durability,  194 

fireproofing,  19] 

hard  and  soft,  191 

strength  of,  193 
Testing  engineer,  the,  263 
Testing  machines,  Emery,  271 

hardness,  286 

hydiaulic,  270 

impact,  279 

repeated  stress,  284 

screw-power,  267 

tension-  compressio  n  -  flexure , 
267 

torsion,  272 

Testing    materials,    chemical    tests, 
264 

commercial,  264 

importance  of,  262 

load  tests,  266 

microscopic  tests,  264 

physical  tests,  264 

research,  266 

sampling,  264 

test  specimens,  265 
Tests,  cold-bend,  287 

colorimetric  for  sand,  208 

of  concrete  aggregate  by  sieving, 
216 

hardness,  286 

impact,  279 

magnetic,  288 


Tests~of  materials,  8,  290 

physical,  264 

repeated  stress,  47 
Thermit  process  welding,  139 
Three-metal  alloys,  159 
Timber,  see  wood 
Tin-copper  alloys,  158 
Titanium  in  steel,  148 
Torsion,  19 

formula,  19 

non-circular  shafts,  20,  21 

indicators,  277 

members,  tensile  and  compres- 
sive stresses  in,  21 
Toughness,  3,  35 
Transverse  shear,  19,  24, 
Triplex  process  for  steel,  105 
Troostite,  133 
Tungsten  steel,  147 


U 


Ultimate  strength,  32 

Uniformity  of  materials,  4 

Uniformly  distributed  stress,  12 

Unit  strain,  10 

Unit  stress,  10 

"  Useful  limit,"  279 

V 

Vanadium  steel,  147 
Veneer,  182 

Voids  in  concrete  aggregate,  209 
proportioning  by,  212 

W 

Water  for  hydration  of  cement,  239 

in  concrete,  224,  234,  246 
Waterproofing  concrete,  250 
Welding,  arc,  139 

applications  of,  139 

electric,  138,  139 

forge,  137 

lusion,  138 

gas,  138 

oxyacetylene,  138 

plastic,  137 

spot,  138 


INDEX 


315 


Welding,  strength  of  welds,  139,  140 

thermit  process,  139 

types  of  weld,  137 
White  cast  iron,  111 
Working  stress,  59,  63 

in  concrete,  249 

for  machines,  64 

for  repeated  stress,  64 

standard,  62 
Wood,  annual  rings,  172 

classification,  170 

decay  of,  184 

defects  in,  180 

elastic  and  proportional  limit, 
176 

grading  of,  181 

grain  of,  173 

hard,  165 

heartwood  and  sapwood,  172 

knots  in,  173,  180 

moisture  in,  169,  178 

plywood,  182 

preservative  processes,  184 

production  in  U.  S.,  167 

quarter  sawed,  170 

relation  of  strength  and  shrink- 
age to  density,  180 

resistance  to  impact,  176 

seasoning,  167 

shear  along  grain,  174 

shrinkage,  169 

slabs,  170 

soft,  165 

spring  wood  and  summer  wood, 
172 

standard  sizes,  170 

strength  of,  174 

strength  of  large  pieces,  178 


Wood,  strength,  treated,  186 

structure  of,  171 

supply  of,  167 

time  element  in  strength,  179 

uses  of,  165,  171,  185 

varieties  of,  165 

veneer,  182 
Wrought  iron,  characteristics,  83 

charcoal  iron,  85 

chemical  composition,  84 

cost,  84 

definition  of,  80 

distinguished  from  steel,  81,  84 

importance  of,  80 

knobbling  process,  85 

for  repeated  stress,  53  • 

merchant  bars,  82 

muck  bars,  81 

phosphorus  in,  81 

puddling  process,  81 

uses  of,  80 

see  also  puddling  furnace 


Yield  point,  3,  31 

determination  of,  279 
significance  of,  32 
stress  beyond,  35 

Z 

Zinc  chloride  for  preserving  wood 

1-84 

-copper  alloys,  1 57 
production  of,  155 
properties  of,  156 
rolling,  155 
uses  of,  156 


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